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I saw F-stop, F/stop and F stop, which is the best one ?
I don't like your edit it's a ratio for example : Diaphragm diameter = 12,5 mm Focal length = 100 mm -> 100/12,5=f/8 Diaphragm diameter = 25 mm Focal length = 50 mm -> 100/25=f/4 Is it logarithmic ? Not really f/4 means twice more light on the film that f/8. and 8=2x4.
I think so: a set of f stops are (by convention) particular f-numbers that form a geometric series with a factor of 2 of exposure as the ratio. The Anome
I think so, although apparently there used to be aperture plates called 'stops' according to one reference. If we ignore the f-number markings, the f-stops make a logarithmic scale of exposure value. Given this interpretation, you can then think of taking a half-step along this scale, to make an exposure difference of "half a stop". The Anome
Suite or serie ? Which one is correct English ? Ericd
I was wondering the same thing. I.e., it says "geometric series" and I wondered if "geometric sequence" should be used instead. A series is a sum, but many persons not schooled in mathematics use "series" when they ought to use "sequence". -- Mike Hardy
Correct word is "suite" in French. Geometric sequence is more correct ? Ericd
French "suite" corresponds to English "sequence", not to English "series". Mathematical illiterates often write "series" where "series" is wrong and "sequence" is right. -- Mike Hardy
This seems OK now. See Geometric progression Ericd
"Geometric progression" will also serve. -- Mike Hardy
I should probably move these f-stop demonstration pics to a separate page? I think it's useful to see the difference, maybe I'm wrong, but I also think that I should take these off for now because many are still on dialup. Opinions. Koyaanis Qatsi
(this is a translation) User Lochkarte wrote on http://www.heise.de/newsticker/foren/go.shtml?read=1&msg_id=5833355&forum_id=57389
concerning: http://en.wikipedia.org/wiki/F-number
Change of September 11. 2003, 02:09PM.
In the article it is written that the f-number (focal number) gets calculated by the diameter of the diapragma and the focal length.
Wrong!
The diaphragma most often is somewhere inside the objective. The lenses before the diaphragma therefore will map it. This map is called the entry pupil. The diameter of this entry pupil is the value crucial for the focal number! (translation end)
Where is the F-stop.jpg image gone ? Ericd 22:47, 27 Jan 2005 (UTC)
Perhaps these could be changed to show the site's title, rather than just their URL. 137.186.22.183 20:28, 15 November 2005 (UTC)
How is eg. "f/8" read? Could put this in the second paragraph or so. TomViza 17:15, 22 December 2005 (UTC)
Focal ratio is a generalized optic term, NOT exclusively associated with cameras or even refracting lenses. It's also used with and applicable to Newtonian and Cassegrain reflecting telescopes, mirror telephoto lenses, and even hand held bathroom mirrors.
That needs to be described, along with a graphic depicting the focal ratio of a parabolic mirror. Something like this, but with arrows showing diameter and focal distance: [3] If nobody else does it, I'll try to add something when I get time. Joema 23:23, 7 January 2006 (UTC)
f stop in a digital camera relates to the size of the CCD/CMOS sensor as much as focal length in a 35mm camera relates to the ratio of the projected image versus the diagonal dimansion of the film frame. This is not covered in this article at all adn should be. I'm not the one to describe this in any detail unfortunately.
-- 195.171.114.69 16:57, 1 February 2006 (UTC)James Laver jameslaver@cybermaps.co.uk 1st Feb 2006
The more a DSLR is stopped down the less forgiving it will be of (dreaded) sensor dust. —Preceding unsigned comment added by 209.134.164.135 ( talk) 20:28, 7 October 2008 (UTC)
I reverted several edits tonight, for several reasons:
The concept of "all else being equal" is often misapplied in depth-of-field reasoning, but clarifying with "For a given image diagonal" (as 83.67.33.215 did, and I reverted) does not make that statement about DOF more correct. The presumption might be that when changing f-number, as many as possible of other variables are kept constant (image format diagonal and hence circle-of-confusion diameter limit, lens focal length, subject distance, absolute aperture diameter). But since they can't ALL be kept constant if f-number is to change, something has to give. Imagining format diagonal changing seems silly, since that won't change the f-number, so why specify that it is constant? The more important thing is that you probably mean to fix the focal length and vary the absolute aperture diameter, as opposed to the other way around. In fact the statement about higher f-number giving higher DOF would be false if the aperture diameter were held fixed and the focal length varied to vary the ratio f/d. So if anything, the clarification should say "for a given focal length". But would anyone really be confused the way it is? Dicklyon 17:41, 15 May 2006 (UTC)
Srleffler, thanks for taking out this external link again [4]. To whoever added it, please tell us why. Is there anything informative in it? I found it bizarre. For example, the section entitled "Where do these f-stop numbers come from?" has a long strange discussion of the inverse square law of light, with no suggestion of how that might be relevant -- and nothing else! Dicklyon 00:54, 30 June 2006 (UTC)
Well, he does admit that "I have not found the following information presented in any photography books that I have read." Somehow, this whole diatribe about f-numbers managed to not mention focal length. So that can't very well be the distance he was thinking about squaring, can it? Dicklyon 03:03, 30 June 2006 (UTC)
"For example, if the focal length is 16 times the pupil diameter, the f-number is f/16, or N = 16.": shouldn't N = f/16, and not 16?
"ISO speed is defined only in one-third stop increments, and shutter speeds of digital cameras are commonly on the same scale in reciprocal seconds. A portion of the ISO range is the sequence
4, 5, 6, 8, 10, 12, 16, 20, 25, 32, 40, 64, 80, 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, 1250, 1600 "
Well no this not an ISO range its an ASA range. Ericd 20:57, 17 July 2006 (UTC)
The International Organization for Standardization (ISO) has a performance-based ISO speed standard for digital cameras, just as they have for film. ISO Standard 12232:2006 ("Photography -- Digital still cameras -- Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index") defines ISO speed in terms of the amount of light needed to achieve a certain "quality" in the sense of a per-pixel signal-to-noise ratio.
From Film_speed#Digital_camera_ISO_speed_and_exposure_index : "However, this standard ISO speed "rating" for a digital camera is not necessarily very related to the ISO "setting" or "exposure index" used on the camera." Of course they are ISO standard for nearly everything... The dynamic range of a CCD is so narrow compared to film that I don't believe the method has much in common with the method used for film.... Ericd 23:19, 17 July 2006 (UTC)
I didn't match it. However I think shutter speed is a bit out of topic here, I don't see any relationship between shutter speed and third stop increment while the relationship between DIN and third stop is obvious IMO. Ericd 23:31, 17 July 2006 (UTC)
Why not draw the line some place like f/1.2? A long list here pretty much dilutes the article and the point of the section. Dicklyon 00:03, 18 July 2006 (UTC)
Well IMO a fast lens is somewhat relative to the focal length... I've tried to improve this list but I'm still not sure it is at the right place. This is a list of some well-known photographic or TV lenses. But there is a lot of (mostly unknown) faster lenses for special purpose (old photocopiers for instance). I think we could move this to photographic lenses ? Ericd 00:33, 18 July 2006 (UTC)
Yes, I know, a "fast" long lens might have a higher f-number than a "fast" short lens. But the text says fast means low f-number, and if you want to change that meaning you really need to say so. Since someone introduced this section to mention some lenses around f/1, I think we ought to hold to just a few very-low-f-number lenses. Or get rid of it. Dicklyon 03:38, 18 July 2006 (UTC)
I was hunting down the term used with astronomical telescopes f-ratio and found it has no page but that this page covers it. It does have a sub-heading on the page Optical telescope -- > Focal length and f-ratio. I am thinking f-ratio is just short hand for focal ratio and that f-ratio should be directed here. If so it should mention somewhere on this page that "f-ratio" is a variant term. As noted above it looks to me like the intro should be more optically orientated (i.e. it should be more inclusive of other uses and not just "photography" with a photographic illustration). Halfblue 21:33, 5 October 2006 (UTC)
Some more stuff I am noticing:
Re: Telescopes: "the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image)" This statement in not 100% true. When imaging self-illuminating point sources at (virtual) infinity (those things we call stars) focal ratio has no effect on brightness. All this would work out better if this article was renamed F-number (photography) and maybe a new article was written that is more inclusive of the optical term "Focal ratio". Halfblue 21:54, 5 October 2006 (UTC)
"theoretically" means I am taking into account focal ratio (which this article is about), and not taking into account diffraction (which this article is not about).
Here is the problem. You seem to be going out of your way to find instances where the wording of that whole section can be true. Citing diffraction as the basis will make it sound true... but that is not the same as writing a true article. An Encyclopedia should be at least two things---> factual and useful. When I come across the term f-ratio in an article about astronomical telescopes it’s going to take me here. The article then tells me that each numerical value is "halving of the light intensity from the previous stop". Is this factual? No…. it does not have that effect on astronomical objects. Is it useful? No... by definition something that is not factual is not useful. So there is a problem with the wording "telescopes and binoculars may have a fixed aperture, but the same principle holds". What "principle" are we talking about here? By the way this article is written it’s obvious, the definitions that come after the statement are describing "the principle". If we have one instance where a statement is wrong---> the statement is wrong.
Here is a "to-do". When someone is thinking of buying a telescope this Christmas it would be useful if there was an article what would tell them that an 6" f15 Maksutov telescope does not produce an image that is 10 stops dimmer than a 6" f5 Newtonian. Ripping appart this article about f-numbers in photography may not be the most efficient way to do it. Like I said originally we could probably use a more basic article Focal ratio (optics) that gives the basic theoretical principles and then links out to more relevant useful real world aplications like f-numbers. Halfblue 12:18, 7 October 2006 (UTC)
And your f/15 scope is only about 3.1 stops dimmer than f/5, not 10 stops. But, I take your point, if it's the same diameter, it is "dimmer" only in the sense of less total light in the image due to having a smaller field of view, not in the sense of gettig less light per star. At least it's not totally inconsistent with the photographic use, which says the light per area will be less, which it is if averaged over a given size image field when looking at a uniform star field. Dicklyon 15:30, 7 October 2006 (UTC)
And one more thing; you complain: "The article then tells me that each numerical value is "halving of the light intensity from the previous stop". Is this factual? No…." Actually, with a telescope of a fixed image field size, such as a piece of film, or a given size exit pupil coupled to your retina, this is completely true, whether you "stop down" by decreasing the aperture or you choose a longer focal length with the same aperture, if the object field is "uniform" or has stationary statistics. In the one case, you reduce light from a given object field, and in the other you spread the light out so that only a fraction of the star field gets imaged to your image field. Understand this first if you want to add something that clarifies it. Dicklyon 15:36, 7 October 2006 (UTC)
Finally, there's diffraction. See the section on image quality effects. It makes no sense to have an article on f-number without a discussion of diffraction, since that's the first-order thing that limits your image resolution; this is true especially in telescopes. To say theoretically while ignoring diffraction, where diffraction is the dominant effect as it is in determining the image area of point sources, is just nonsense. Dicklyon 15:40, 7 October 2006 (UTC)
Halfblue 12:59, 20 October 2006 (UTC)
Just a thought: stars are not point sources. They are, in fact, very large extended objects. If you were not limited by diffraction, the size of the image of a star would scale with focal length the same way any other image does. The point source model works only because you are below the diffraction limit, so the size of the image of the star does not scale as predicted by geometric optics. The statements in the article about the properties of f-number are true whenever diffraction is negligible, and false otherwise. Point taken, though, about the need for this article to be useful to astronomers as well as photographers and optical engineers. Some reworking of the text may be required to accommodate everyone's needs.-- Srleffler 23:36, 20 October 2006 (UTC)
I quote latest development from a section above:
I argue that that is not an apppropriate response. Take a look at the page. It is mostly just repetition of this f-number article, with a nice new telescope picture. I see no reason it can be incorporated here, avoiding such splintering of content. I will revert it back to a redirect, doing my best to merge any new content here first, unless somebody else objects. At present, it is just a place for halfblue's misconception or misinformation, in which he implicitly assumes that in varying the focal ratio, the only independent variable is focal length, not diameter. Dicklyon 15:10, 20 October 2006 (UTC)
Since someone beat me to reverting it, so I went back and copied the content, corrected it, and put it in a new section in this article. It's a bit redundant, but that's probably OK. Dicklyon 20:28, 20 October 2006 (UTC)
I see the article has been moved to f-number. Here is the reason for the article Focal ratio---
The problem I see with f-number is not a problem with the article its self (although it does have some fixable errors), it is its place in terms of an encyclopedia. To repeat something I said somewhere else--- There is an old Microsoft joke [5] where a software engineer gives an answer that is 100% correct and relatively useless. I get that feeling when I try to link to f-number.
and has a diagram of standard photographic stops opposite that statement. In other words this is an article about the photographic application of "focal ratio" more than the article describing the basic concept of "focal ratio". The lead in paragraph is not a good description of "focal ratio" as it applies to mirrors, radar/radio telescopes, and parabolic sound reflectors, ect. That could mostly be fixed by replacing the word "lens" with the word Objective (optics) but it shows the bias in the article.
I don't know if there is a wiki term for this but "f-number" is getting to be a mega-article. It is trying to describe everything under the sun that has to do with the ratio of stop to focal length and doing it poorly. I was noticing this before with Aperture. If you link Optical telescope - Focal length and f-ratio to f-number the article tells you a few things that are flat out wrong re: telescopes, stellar point sources, and focal ratio such as:
So--- when we send people to f-ratio when they click a link with the question "I wonder what "f-ratio" does to these different types of telescopes?". Coming into the top of this article they are not going to get an immediate answer (and they are going to say "wait a minute... this is about photography"), Then they are going to get a bogus answer, then, if they preserver, scanning down what is getting to be a long article they may find the newly added content on telescopes. Its not the readers fault if they are confused. They have been given an answer that is about 95% correct and (from their laymans point of view) almost totally irrelevant to what they were looking for. I think this is what is being missed and what Wikepedia means when they say Think of the reader. Directly linking them to F-number#Focal ratio in telescopes may work for a wile till someone says "wait a minute.. thats redundant.. lemme clean it up and make that sub title "f-ratio"" and kills all the links (unless there is some redundancy in wiki-linking that I am not aware of).
Some more observations:
That is more than my two cents.. Halfblue 00:14, 21 October 2006 (UTC)
Besides, the stuff you assert is "flat out wrong" is actually correct, as I've explained in various terms. If there's a better way to explain the effects you care about in astronomy, by all means let's add them, as I tried to do in incorporating your focal ratio article as a new section. I see no inherent difficulty in making this article suit the needs of astronomy, microscopy, and photography. Just add the material as suitable. Dicklyon 06:11, 21 October 2006 (UTC)
Halfblue, I did a quick GBS and found a source for your idea: book page. However, the statement that comparing two 8" scopes the image brightness is not at all dependent on focal ratio is restricted by "if the two scopes are used at the same magnification", which changes it completely, into something that is now true. Without the qualifier, one might naturally assume that brightness referred to equal treatments of the focal plane, e.g. same film or same CCD or same eyepiece, in which case you need some other restriction, such as the total optical power in the spot even as its size changes, to make it true. But for either of these, you also need to make it clear that the diameter is being held fixed, and the focal length is what you are varying. Otherwise, claiming brightness independent of focal ratio is just obviously wrong, in general, for any definition of brightness. Dicklyon 06:20, 21 October 2006 (UTC)
Sorry for slow replies.... the ol' Wikipedia hobby has to take a back seat to life sometimes. Each thing gets one article, no matter how many different terms there are that describe it gets you to the heart of the problem with the f-number article. f-number is not the "thing". It is a number resulting from the thing. The thing here is Focal ratio... f over D. f-number is the results of that equation. Articles get renamed or redirected to their more basic form all the time. In this case we got the redirect going the wrong way. Setting up something like focal ratio (astronomy) is not really a fix because focal ratio is not limited to astronomy... or even the optical spectrum. f-number (with a few minor corrections) is a good article on the applications of focal ratio but it would take a lot to make it inclusive of everything about focal ratio (the intro paragraph needs to be totally rewritten for a start). All of that would be tackling the symptom but not the cause... an article with the wrong name. The simple fix is to set up a page for Focal ratio noting that there are many different applications of the principle (which I have already done) and make some minor edits (and maybe even a name change) to f-ratio to note it is about applications of a more basic principle. Halfblue 12:52, 26 October 2006 (UTC)
This section may end up tying together several threads begun in sections above. I see that someone has created a new article on lens speed. At first glance, that might seem to be redundant with this article, but I think it is actually a good idea. As constructed, that article focuses on the concept of "speed" of a lens, and its role in photography. It eschews the technical details, and uses f-number mainly as a benchmark of "speed". That is a nice complement to this article, which has all the technical detail that is so important to optics. f-number is probably too technical for a reader who is an amateur photographer with little interest in optics. I think we should maintain this separation, and ensure that the links between the articles guide photographers with an interest in more technical detail here, and those with no interest in it there.
We may also want to move some material between the two articles. For example, I moved the section with the list of fast lenses to the other article. It is a much better fit there than here, both conceptually and in terms of who is likely to read which article. One option to consider: we might want to deliberately reduce the focus on photography in this article, and make it more aimed at coverage of f-number in general (i.e. encompassing optics, astronomy, photography, etc.), leaving the lens speed article to cover the aspects of particular interest to photographers. I propose this with some doubt, however: it's not clear to me that that is the way to go.-- Srleffler 05:17, 10 November 2006 (UTC)
Hello,
The
Barry Lyndon articles states that Kubrick used a NASA f/0.70 focal ratio. Problem is, I don't have a clue what it means out of technical thing. Could someone give comparison examples from real world? Like how far can it see, how dark can it see, what is the animal that comes closest to this, and so on. Thank you.
David Latapie (
✒ |
@) 08:00, 29 November 2006 (UTC)
I read your definition of F-Number, and I'm sorry to inform you that there is a mistake using the term "square root of 2." The language states "....sequence of the powers of SQRT(2) (or 2^0.5)...." You should correct this to say ...sequence of powers of 2^x with x=0 -> x=n in increments of 1/2 i.e. 2^0, 2^0.5, 2^1.0, 2^1.5, 2^2....Etc. Therefore 2^2 would correspond to an F-Number (or F-Stop) of f/4. 69.44.134.230 16:49, 29 November 2006 (UTC)
I have another mathematical-interpretation question. The article seems to consider that an f-number of f/4 is smaller than f/8. From a mathematical expression however, if f is constant, f/8 is a smaller number than f/4. Unless I am forgetting/misinterpreting something here, I think this is very confusing. It may very well be that in the photographic world this is the way they consider it, but in that case I would mention this confusion in the article. -- Patrickdepinguin 21:13, 26 May 2007 (UTC)
That's right, the f-number 4 is smaller than 8, and the aperture f/4 is larger than the aperture f/8. It's not a confusion, but it means that ordering of f-numbers and apertures need to be made clear. Is there some place in the article that is not clear? Dicklyon 21:49, 26 May 2007 (UTC)
I believe that this is naturally confusing. In photography, one often says "increase" or "decrease" regarding aperture or f/stop. In that case, "increase" means increasing f/#, and so decreasing #. I have never known in photography the use of the words "increase" and "decrease" to correspond with the #, but always its reciprocal. Not that the article is wrong, but it is hard to argue with tradition. Gah4 ( talk) 08:40, 26 September 2011 (UTC)
The new table has no generality, and serves only to illustrate that each full stops cuts the area in half. This is easy enough to see without spending a big table on it, in my opinion. I vote we remove it. Srleffler has already tagged it as questionable. Anybody think it is worth keeping? Dicklyon 05:59, 30 December 2006 (UTC)
I propose we remove this talk fragment. Since the sheet doesn't exist on Wiki, the F/stop page doesn't refer to it, and I'm not planning on adding it again. -- GordonMcKinney ( talk) 01:58, 16 May 2008 (UTC)
F-number is a technical term used to describe the amount of light allowed through a lense system.
The term was originated when photographers used cameras that allowed the photographer to view the scene before exposing film. In order to make the preview as bright and easy to see as possible, the photographer would open the lense aperture as large as possible. Then, before exposing the film, the photographer would close the aperture to account for film speed, motion, and depth of focus. Eventually, cameras opened and reduced the aperature automatically, and one only set the size to which the reduction would be stopped. The aperture starts open and is "stopped down" to the appropriate diameter just prior to exposing the sensor.
Synonyms of f-number have been introduced over the years, many of which survive today. These include: focal ratio, f-ratio, or relative aperture [1].
The focal length of the lense system is used because the intensity of light is concentrated at higher magnifications. Focal length is a simple way to describe the magnification capability of a lense. Many people have an intuitive grasp of this because it can be demonstrated by burning paper with a magnifying glass in direct sunlight. Covering the lense partially prevents the focused beam from burning the paper because the intensity of light is reduced.
Aperture diameter defines to the amount of light allowed through a lense system, and this is related to the length of time the sensor must be exposed to the in-coming light. The relationship between aperture diameter and exposure time is often quoted as lense speed.
Why is focal length important? How can that be incorporated into this article? neffk 15:06, 26 June 2007 (UTC)
I've had time to think and re-read the text of the article and what I tried to contribute. Initially, I was intimidated by Dicklyon, but I think maybe I should have been more bold about discussing the changes I proposed. I see that many of the points were not appropriately addressed. One of the reasons for reverting my lead was an appeal to tradition: "...what's wrong the rather precise way we had it before?" That's against Wikipedia policy, as I understand it; Wikipedia:BRD. Anyway, see the items below below. neffk ( talk) 16:34, 25 June 2009 (UTC)
The relative aperture of a lens in photography is a ratio of two numbers. What numbers are these?
1) is the diameter across the lens opening (the aperture) (Really the effective aperture - which is what can be seen in actual fact by moving the eye from side to side - multi-element lenses will give a different value than the actual diameter)
2) is the focal length of the lens.
The effective aperture (f stop) is supposed to give the same figure for big cameras and small cameras, for big lenses and small ones.
f/11 means the diameter of the lens will divide into its focal length eleven times.
This can also be expressed as 1:11 where 1 is the focal length of the lens. (Lenses are marked say 1:11) This is known as the maximum relative aperture of the lens.
If we write f/11, f means the focal length of the lens and the slash means "divided by".
The relative aperture gives the amount of light reaching the image plane (eg the film) because the inverse square law says that light increases (or decreases)by the square of the distance. Halving the distance to a light source will not double the light it will quadruple it. And increasing the distance by three times will reduce the light by the square of three = 9. Only one-ninth of the illumination will get there.
In a camera, the relative aperture takes this into account. RPSM 16:22, 11 November 2007 (UTC)
Hi, I was looking at other articles, and some people in other places are getting themselves tied up in knots. Here is from article Camera
Various Cameras: An Agfa Brownie, Polaroid Land Camera, and Yashica 35 mm SLR The size of the aperture and the brightness of the scene control the amount of light that enters the camera during a period of time, and the shutter controls the length of time that the light hits the recording surface.
Well - it's the value of the relative aperture innit?
I only have simple math, and I was thinking of explaining f/number in the simplest terms possible to make it available to the youngest and least technical readers (like me).
In optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture[1]) of an optical system expresses the diameter of the entrance pupil in terms of the effective focal length of the lens.
in terms of is no doubt correct, but will it be understood by people with only + - x and ./. ?
I prefer the focal length divided by the effective aperture.
What's the effective focal length?
Is that the actual focal length in use, or what. How can you reduce the explanation to be as simple as possible for even young readers?
You use the term "relative aperture" I think this is the correct term. F stop etc is everyday talk.
This article is fine. I will try to find the other places in wiki where they go into paroxysms of mathematics without starting from basic principles. Best RPSM 16:47, 11 November 2007 (UTC)
Did you do the article on '''Aperture''' as well?
That looks ok.
I'm just into very simple explanations for beginners without too much math. It doesn't have to be like that on Wiki.
At some point the relationship between basic definitions (focal length, f/stop, etc) and how the math works forces non-mathematicians to say: now I have to get the hang of this in mathematical terms.
Of course light captured by a lens is one thing, and light received at the image plane another.
Are we going to go into foot-lamberts and lux as well?
And what is the math for exposure meters and such? RPSM 17:04, 11 November 2007 (UT
f-number is defined as the ratio of focal length to the diameter of the entrance pupil.
This is what you said in chat above: I like that better.
Of course, when you look at a lens it has on it either F/4.5 or else 1:4.5 as well as the focal length expressed as f= 7" or f=5cm.
I just thought that beginners could relate theory to practice by showing that these are mathematical expressions that give a key to the theory. ¨¨¨¨ —Preceding unsigned comment added by RPSM ( talk • contribs) 18:09, 11 November 2007 (UTC)
One origin of the word stop is from Waterhouse stops, which, as you know, are slips of metal with a hole in. Let's say you are only using one stop all the time, you stick in the stop and you don't have to read any scale or anything. And you can directly measure the diameter on a rule. They are used with lenses for large field cameras or studio cameras. In that case, you did not have to go round to the front of the camera and peer at the aperture scale.
The other type - normal today - is the iris diaphragm. (diaphragm alone is used for a number of things in other fields) RPSM 18:15, 11 November 2007 (UTC)
We don't have a simple explanation of focal length. I always say it is the distance from the optical centre of the lens to the image plane when the camera is focused on infinity. Simpler: When the camera (lens) is focused on a distant object - a range of mountains or the sun, the rays of light entering the lens are parallel, or very nearly so for our purposes. Then the distance from the optical centre of the lens to the film (image plane) is the same as the focal length of the lens. http://www.paragon-press.com/lens/lenchart.htm is a simple explanation. However I like the full-blown technical explanation as well, and I want relative focal length back somewhere. I did a photography exam once and one question was: What is the difference between relative aperture and effective aperture? and it wasn't in the books we had. Now I get it after seeing it here. Thanks. RPSM 13:58, 12 November 2007 (UTC)
Sorry, effective focal length I'm learning all the time. RPSM 14:45, 13 November 2007 (UTC)
First two references are for opticians (eye doctors), third one is from Slovenia: http://www.nfos.org/degree/opt11/module_13a.html http://www.nfos.org/degree/opt11/module_13b.html "So now we come to the second most used formula in optical theory -- Prentice's rule. This formula gives us an approximation of the amount of prism present at any given point on a lens based on the dioptric power of the lens and the distance that the point is from the optical center of the lens." [I think they mean the optical axis (?)] Next reference is better: http://lrv.fri.uni-lj.si/~peterp/publications/prl06.pdf "The optical center within the camera represents the point in which all the light rays that form the image intersect if the lens is approximated with a single thin lens." It looks like these people were having a hard time trying to find the optical centre - Is that what you mean by saying it is undefined? RPSM 14:50, 13 November 2007 (UTC)
Thanks RPSM 08:01, 14 November 2007 (UTC)
I've seen image-space f-number = effective focal length / entrance pupil diameter and paraxial working f/# = 1/ (2 tan U') where U' is the angle between the entering marginal ray and the exiting marginal ray. Also, working f/# = 1/(2 sin U'). Should this article cover these topics or should they get their own pages? 155.212.242.34 19:21, 3 December 2007 (UTC)
The article says f/number measures lens speed. It seems to me that effective lens speed will have more to do with the entrance pupil area times the spherical angle of view divided by the area of the sensor. That is, how much light comes in divided by the area it is projected against. Furthermore, in theory I could imagine a lens with a huge entrance pupil and a narrow angle of view but a tiny exit pupil and a tiny back focal length. Obviously within one lens, changing the f/number changes the amount of light accordingly, but between lenses, what does it really measure? 155.212.242.34 ( talk) 17:19, 27 December 2007 (UTC)
Here are a couple of good books about all this. Lee speaks of both the entrance-pupil-based and exit-pupil-based approach to f-number, and the fact the pupil magnification may make them not equal; for object at infinity, they will be equal, but otherwise it gets complicated. Also, note that the other book uses positive magnification convention. Dicklyon ( talk) 06:56, 25 January 2008 (UTC)
Now that we've got m negative again, in agreement with the cited source, and "working" f-number being N*(1-m), maybe we should reconsider. When I search books for "working f number", I get pretty much nothing. But with "effective f number" I get quite a lot of hits, and at first glance it appears that more of them use the photography convention of unsigned m, rather than the optics convention of algebraic m. The forumula with 1+m (or 1+M) is common in these books. Should we consider re-wording, and at least give the two conventions equal weight? Dicklyon ( talk) 01:25, 25 January 2008 (UTC)
I vote for the photographically conventional positive / unsigned magnification. This case is explicitly mentioned in the magnification article too. So I would suggest to change all photographically related occurrences of M and especially this odd N*(1-M). Alex.altmaster ( talk) 17:05, 29 January 2008 (UTC)
There is a discussion on a change in typography for f/ happening at Template talk:F/. Please join in if you care about f/ vs. f/.-- Srleffler ( talk) 05:39, 3 August 2008 (UTC)
This sentence from "Effects on Image Quality" doesn't make sense to me. Is it missing a phrase, or am I just confused? "Therefore, reduced-depth-of-field effects, like those shown here, will require smaller f-numbers (and thus more complex optics) than do larger format cameras." Billgordon1099 ( talk) 16:55, 2 September 2008 (UTC)
maybe a footnote on 3.3 would be in place? It should really be 3.2, since 2^((10/3)/2)=3.1748, but probably for some historical reasons the 3.3 from the half scale was inherited.
There is even a note on 3.2, but it might be confusing that 3.2 is not to be found on the scale.
Cheers,
Bo —Preceding unsigned comment added by Babbletower ( talk • contribs) 22:17, 20 September 2008 (UTC)
I don't know if there is a historical significance to f3.3 but in the half-stop table, 2^[(7/2)(0.5)] ≈ 3.36 = 3.4 by round off rules. —Preceding unsigned comment added by 50.46.174.62 ( talk) 16:48, 23 April 2011 (UTC)
I had some minor issues with the current intro text; eg. the confusing use of "effective" aperture ... and the unintentional suggestion that "f-number" was the preferred term for relative aperture in the scientific field of optics ... minor stuff, but I drafted a "new version" for your approval ;)
It might also be helpful/illustrative to point out the the focal ratio on many zoom lenses includes two values for maximum aperture; eg "1:3.5-5.6", the first referring to max aperture at shortest focal length (wide end), the second at longest (full zoom). AFAIK, the maximum physical aperture itself does not change relative to focal length, but the relative aperture does. -- Redbobblehat ( talk) 06:31, 18 February 2009 (UTC)
There seems to be disagreement whether the focal length of the human eye is 17mm or 22mm. Look, for example http://hypertextbook.com/facts/2002/JuliaKhutoretskaya.shtml .
Is there anyone who can explain the discrepancy in detail?
I think it would be appropriate to add (or link to) some information about the variable light sensitivity of the eye. Seems like this kind of information should be available somewhere in Wikipedia, but I didn't find it. neffk ( talk) 13:04, 26 June 2009 (UTC)
This section says the human eye in the dark opens to about f/2.1. Is this true? That would mean that certain lenses would actually be brighter than the human eye ( Sigma 50mm f/1.4, Nikon 35mm f/1.8G, Canon 50mm f/1.4) and that doesn't seem right. -- The High Fin Sperm Whale ( Talk • Contribs) 05:14, 20 December 2009 (UTC)
This article needs some discussion of the manufacturing precision/glass quality required to achieve low f numbers. When I was getting into photography, I got the sense that low f numbers indicated high-quality lenses but wondered why they didn't just allow the aperture to open up further. Similarly, you see old lens designs listed as being an "f/16 landscape lens" and that eventually lens design allowed for f/2.8 lenses and faster. There's obviously potential issues with vignetting, but beyond that, I asked myself "Why can't you take an f/16 lens and make the aperture bigger?"
The answer, as I understand it, is threefold. First is vignetting: Making a faster version of some designs would lead to negative thickness at the edges of elements. Second is the lens design itself, that fundamentally aberrations would start to dominate if you opened a lens up, so while you could make a fast version of an old slow lens, even ideally constructed it would produce very soft images, even at the center of the image. Third is manufacturing quality, that fast lenses are more sensitive to imperfections in surfaces and alignment, since for an entire pencil of rays to come to diffraction-limited focus, the entirety of every surface that pencil crosses must be the right shape to within wavelength tolerances across those areas.
Do those three sound right? If so, let's add some mention of it. —Ben FrantzDale ( talk) 12:48, 7 January 2011 (UTC)
The diagram showing how two lenses with different focal lengths but the same aperture setting, and different aperture diameters will produce the same illuminance in the focal plane appears to be incorrect.
I know that the caption states that "Focal lengths and aperture sizes are not to scale", but if it is drawn to scale, it looks a little different. See here for a diagram drawn to scale, where it shows that the lines meeting the aperture diameter actually meet at a point on the focal plane. Martybugs ( talk) 12:57, 8 January 2011 (UTC)
In this diff, User:Glockenklang1 prefers this new lead section:
The ratio of focal length and aperture of an imaging system is dimensionless and defines the F-number [1]
F is identical to the inverse of the relative aperture( [2]) and is referred to also as focal ratio, f-ratio, f-stop, "f/#".
More cautiously the aperture has to be a linear measure of the entrance pupil of rays forming the image, thus the effective aperture. The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.
The inverse of F-number is often used in astronomy.
The Iluminance of the image goes inversely with the square of the F-number
A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 135 mm focal length lens with a setting of f/4 will have a pupil diameter of about 33.8 mm. The 135 mm lens' f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.
Other types of optical system, such as telescopes and binoculars may have a fixed aperture, but the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image).
to this old stable version:
In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture [3]) of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter. It is a dimensionless number that is a quantitative measure of lens speed, an important concept in photography.
The f-number (f/#) is often notated as and is given by
where is the focal length, and is the diameter of the entrance pupil. By convention, "f/#" is treated as a single symbol, and specific values of f/# are written by replacing the number sign with the value. For example, if the focal length is 16 times the pupil diameter, the f-number is f/16, or . The greater the f-number, the less light per unit area reaches the image plane of the system; the amount of light transmitted to the film (or sensor) decreases with the f-number squared. Doubling the f-number increases the necessary exposure time by a factor of four.
The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.
A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 135 mm focal length lens with a setting of f/4 will have a pupil diameter of about 33.8 mm. The 135 mm lens' f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.
Other types of optical system, such as telescopes and binoculars may have a fixed aperture, but the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image).
I think this is too much to change without talking about it. Definition, calling it "F", lead image caption, initial section titles, lots of text, etc. Some of these changes may be OK, but it's not clear what problem he's trying to solve. Opinions?
Dicklyon (
talk) 01:03, 6 February 2011 (UTC)
The ratio of focal length and aperture of an imaging system is dimensionless and defines the f-number
N is referred to also as focal ratio, f-ratio, f-stop. Optical Lenses are very often denoted by "f/N". Example: a lens with f=400mm D=100mm the formula gives N=4 and would be denoted by f/4.
More cautiously the aperture has to be a linear measure of the entrance pupil of rays forming the image, thus the effective aperture. The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.
The Iluminance of the image goes inversely with the square of the f-number
A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. The same lens set to f/8 will have D=12.5mm and supply only 1/4 of the amount of light for exposure. Another lens with f=135 mm setting of f/4 will have a pupil diameter of about 33.8 mm. The 135 mm lens' f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.
Other types of optical system, such as telescopes and binoculars may have a fixed aperture, but the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image). -- Glockenklang1 ( talk) 21:24, 11 February 2011 (UTC)
I agree with Jeff. I still don't see what problem is being addressed by these proposed changes. The lead sentence gets away from normal style, and mentions simple tangents like "dimensionless" before mentioning the topic and the alternative terms for it, which conventionally comes first. The other names shouldn't be indirected through N, and we don't need an example in the lead paragraph. It's not clear where the "caution" thing is coming from, as there's no danger apparent; the entrance pupil is already mentioned, as opposed to the more ambiguous term "aperture" (which is never intended to mean the aperture stop's diameter, I'm pretty sure). Dicklyon ( talk) 06:34, 12 February 2011 (UTC)
The current formula for Working f-number, though correct, is uncommon in photography, where inversion of the image is typically ignored, and magnification is always taken as positive (e.g., Ray 2002). I think this section would be more clear if we mentioned the photographic convention and included the formula as it is given in most books on photographic optics, where it’s often called the effective f-number:
Many books on photographic optics, including Ray (2002), distinguish between the effective f-number and the marked f-number (the one given by the formula at the beginning of this article and usually given for camera lenses). Perhaps we should mention this as well. JeffConrad ( talk) 10:15, 13 February 2011 (UTC)
Umm . . . wudja believe
JeffConrad ( talk) 14:19, 13 February 2011 (UTC)
My ongoing additions and clarifications to this article in regard to the vendor specific usage of f-number scales have been reverted by Dicklyon two times. My lastest in-edit version can be inspected here:
http://en.wikipedia.org/?title=F-number&diff=438918941&oldid=438902195
Dick, you wrote that my additions were unsourced and original research and likely untrue. While I cannot claim to be free of errors, the information I gave was carefully compiled and cross-checked from various sources over the years, and in those cases where I found diverting infomation I either omitted the info or added a ? to indicate this. The information on Minolta, Konica Minolta, and Sony A-mount cameras is definitely correct, as I have verified the information with my own large set of cameras and lenses (1985-2011). Even the values outside the range of currently existing lenses have been verified by inspecting the cameras on protocol level and using "fake" adapters to emulate lenses with properties not yet commercially available. Also, some of the camera firmware images have been reverse-engineered and corresponding display values be found in the firmware. I even gave quite a number of references with detailed background information (including proof photos), however, I did not want to clutter the article with references, that's why I gave most of them in the form of HTML source code comments < ! - - - - > not via < ref > tag. Maybe you missed this? The f-numbers displayed by Minolta, Konica Minolta and Sony A-mount cameras have been consistent over all the years, with basically just one exception: While Minolta bodies never reported an aperture of 1.8 (in 1/2EV or 1/3EV steps), Sony cameras started to report both 1.7 and 1.8 depending on the lens used - this is down to internal rounding. Otherwise, all camera bodies report the same values from 1.0 to 64, and the Konica Minolta and Sony cameras also seem to support values downto 90 (and there are some reports of values downto 128, but I did not included this information, because I could not verify it). The Minolta 1/4EV f-number scale is complete and correct as well - there have been in total three Minolta devices supporting the display of apertures in 1/4EV granularity, and I have verified the displayed values on each of them. There seems to be more inconsistency in the f-number scales used by Canon and Pentax, at least older cameras (such as the T90) reported different values than EOS cameras do, and I also found some different information in Canon service manuals regarding some values. Since I do not use Canon and Pentax cameras myself I have to rely on third-party information here (but several user reports have been used to verify the information as much as possible). I am quite sure that even with some inconsistencies here, it is possible to draw a timeline somewhere when these vendors changed their scales. I do think, that this information is encyclopedic and relevant and useful to the article. The article in the current form teaches typical usages of 1/2EV and 1/3EV f-number scales which in fact do not reflect the typical usage of f-numbers of some of the largest camera manufacturers today (and in the past twenty years or so). That's what I would call "likely untrue" (and it is not sourced in any way). I do think that adding this information to the tables (and not to the text) is the best way to represent it, because readers can then easily compare the f-numbers scales this way. Do you have a better idea? Ideally, I would like to see this article list and compare the f-number scales used by *any* camera manufacturer since ca. 1985 (when cameras started to display f-numbers in 1/2 or 1/3EV steps). In some cases, it might be necessary to list more than one scale per vendor, but it wouldn't harm, since the number of vendors is limited. -- 88.77.195.147 ( talk) 18:43, 11 July 2011 (UTC)
In ZEMAX, there are three f/#s listed in the System Data report: Image Space f/#, Paraxial Working f/#, and Working f/#. I'm trying to understand what they mean and the ZEMAX documentation is a bit thin.
Could someone comment on these? I haven't found a reference anywhere that gives good optical-engineering definitions. —Ben FrantzDale ( talk) 17:05, 4 August 2011 (UTC)
This page is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page. |
This page is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page. |
I saw F-stop, F/stop and F stop, which is the best one ?
I don't like your edit it's a ratio for example : Diaphragm diameter = 12,5 mm Focal length = 100 mm -> 100/12,5=f/8 Diaphragm diameter = 25 mm Focal length = 50 mm -> 100/25=f/4 Is it logarithmic ? Not really f/4 means twice more light on the film that f/8. and 8=2x4.
I think so: a set of f stops are (by convention) particular f-numbers that form a geometric series with a factor of 2 of exposure as the ratio. The Anome
I think so, although apparently there used to be aperture plates called 'stops' according to one reference. If we ignore the f-number markings, the f-stops make a logarithmic scale of exposure value. Given this interpretation, you can then think of taking a half-step along this scale, to make an exposure difference of "half a stop". The Anome
Suite or serie ? Which one is correct English ? Ericd
I was wondering the same thing. I.e., it says "geometric series" and I wondered if "geometric sequence" should be used instead. A series is a sum, but many persons not schooled in mathematics use "series" when they ought to use "sequence". -- Mike Hardy
Correct word is "suite" in French. Geometric sequence is more correct ? Ericd
French "suite" corresponds to English "sequence", not to English "series". Mathematical illiterates often write "series" where "series" is wrong and "sequence" is right. -- Mike Hardy
This seems OK now. See Geometric progression Ericd
"Geometric progression" will also serve. -- Mike Hardy
I should probably move these f-stop demonstration pics to a separate page? I think it's useful to see the difference, maybe I'm wrong, but I also think that I should take these off for now because many are still on dialup. Opinions. Koyaanis Qatsi
(this is a translation) User Lochkarte wrote on http://www.heise.de/newsticker/foren/go.shtml?read=1&msg_id=5833355&forum_id=57389
concerning: http://en.wikipedia.org/wiki/F-number
Change of September 11. 2003, 02:09PM.
In the article it is written that the f-number (focal number) gets calculated by the diameter of the diapragma and the focal length.
Wrong!
The diaphragma most often is somewhere inside the objective. The lenses before the diaphragma therefore will map it. This map is called the entry pupil. The diameter of this entry pupil is the value crucial for the focal number! (translation end)
Where is the F-stop.jpg image gone ? Ericd 22:47, 27 Jan 2005 (UTC)
Perhaps these could be changed to show the site's title, rather than just their URL. 137.186.22.183 20:28, 15 November 2005 (UTC)
How is eg. "f/8" read? Could put this in the second paragraph or so. TomViza 17:15, 22 December 2005 (UTC)
Focal ratio is a generalized optic term, NOT exclusively associated with cameras or even refracting lenses. It's also used with and applicable to Newtonian and Cassegrain reflecting telescopes, mirror telephoto lenses, and even hand held bathroom mirrors.
That needs to be described, along with a graphic depicting the focal ratio of a parabolic mirror. Something like this, but with arrows showing diameter and focal distance: [3] If nobody else does it, I'll try to add something when I get time. Joema 23:23, 7 January 2006 (UTC)
f stop in a digital camera relates to the size of the CCD/CMOS sensor as much as focal length in a 35mm camera relates to the ratio of the projected image versus the diagonal dimansion of the film frame. This is not covered in this article at all adn should be. I'm not the one to describe this in any detail unfortunately.
-- 195.171.114.69 16:57, 1 February 2006 (UTC)James Laver jameslaver@cybermaps.co.uk 1st Feb 2006
The more a DSLR is stopped down the less forgiving it will be of (dreaded) sensor dust. —Preceding unsigned comment added by 209.134.164.135 ( talk) 20:28, 7 October 2008 (UTC)
I reverted several edits tonight, for several reasons:
The concept of "all else being equal" is often misapplied in depth-of-field reasoning, but clarifying with "For a given image diagonal" (as 83.67.33.215 did, and I reverted) does not make that statement about DOF more correct. The presumption might be that when changing f-number, as many as possible of other variables are kept constant (image format diagonal and hence circle-of-confusion diameter limit, lens focal length, subject distance, absolute aperture diameter). But since they can't ALL be kept constant if f-number is to change, something has to give. Imagining format diagonal changing seems silly, since that won't change the f-number, so why specify that it is constant? The more important thing is that you probably mean to fix the focal length and vary the absolute aperture diameter, as opposed to the other way around. In fact the statement about higher f-number giving higher DOF would be false if the aperture diameter were held fixed and the focal length varied to vary the ratio f/d. So if anything, the clarification should say "for a given focal length". But would anyone really be confused the way it is? Dicklyon 17:41, 15 May 2006 (UTC)
Srleffler, thanks for taking out this external link again [4]. To whoever added it, please tell us why. Is there anything informative in it? I found it bizarre. For example, the section entitled "Where do these f-stop numbers come from?" has a long strange discussion of the inverse square law of light, with no suggestion of how that might be relevant -- and nothing else! Dicklyon 00:54, 30 June 2006 (UTC)
Well, he does admit that "I have not found the following information presented in any photography books that I have read." Somehow, this whole diatribe about f-numbers managed to not mention focal length. So that can't very well be the distance he was thinking about squaring, can it? Dicklyon 03:03, 30 June 2006 (UTC)
"For example, if the focal length is 16 times the pupil diameter, the f-number is f/16, or N = 16.": shouldn't N = f/16, and not 16?
"ISO speed is defined only in one-third stop increments, and shutter speeds of digital cameras are commonly on the same scale in reciprocal seconds. A portion of the ISO range is the sequence
4, 5, 6, 8, 10, 12, 16, 20, 25, 32, 40, 64, 80, 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, 1250, 1600 "
Well no this not an ISO range its an ASA range. Ericd 20:57, 17 July 2006 (UTC)
The International Organization for Standardization (ISO) has a performance-based ISO speed standard for digital cameras, just as they have for film. ISO Standard 12232:2006 ("Photography -- Digital still cameras -- Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index") defines ISO speed in terms of the amount of light needed to achieve a certain "quality" in the sense of a per-pixel signal-to-noise ratio.
From Film_speed#Digital_camera_ISO_speed_and_exposure_index : "However, this standard ISO speed "rating" for a digital camera is not necessarily very related to the ISO "setting" or "exposure index" used on the camera." Of course they are ISO standard for nearly everything... The dynamic range of a CCD is so narrow compared to film that I don't believe the method has much in common with the method used for film.... Ericd 23:19, 17 July 2006 (UTC)
I didn't match it. However I think shutter speed is a bit out of topic here, I don't see any relationship between shutter speed and third stop increment while the relationship between DIN and third stop is obvious IMO. Ericd 23:31, 17 July 2006 (UTC)
Why not draw the line some place like f/1.2? A long list here pretty much dilutes the article and the point of the section. Dicklyon 00:03, 18 July 2006 (UTC)
Well IMO a fast lens is somewhat relative to the focal length... I've tried to improve this list but I'm still not sure it is at the right place. This is a list of some well-known photographic or TV lenses. But there is a lot of (mostly unknown) faster lenses for special purpose (old photocopiers for instance). I think we could move this to photographic lenses ? Ericd 00:33, 18 July 2006 (UTC)
Yes, I know, a "fast" long lens might have a higher f-number than a "fast" short lens. But the text says fast means low f-number, and if you want to change that meaning you really need to say so. Since someone introduced this section to mention some lenses around f/1, I think we ought to hold to just a few very-low-f-number lenses. Or get rid of it. Dicklyon 03:38, 18 July 2006 (UTC)
I was hunting down the term used with astronomical telescopes f-ratio and found it has no page but that this page covers it. It does have a sub-heading on the page Optical telescope -- > Focal length and f-ratio. I am thinking f-ratio is just short hand for focal ratio and that f-ratio should be directed here. If so it should mention somewhere on this page that "f-ratio" is a variant term. As noted above it looks to me like the intro should be more optically orientated (i.e. it should be more inclusive of other uses and not just "photography" with a photographic illustration). Halfblue 21:33, 5 October 2006 (UTC)
Some more stuff I am noticing:
Re: Telescopes: "the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image)" This statement in not 100% true. When imaging self-illuminating point sources at (virtual) infinity (those things we call stars) focal ratio has no effect on brightness. All this would work out better if this article was renamed F-number (photography) and maybe a new article was written that is more inclusive of the optical term "Focal ratio". Halfblue 21:54, 5 October 2006 (UTC)
"theoretically" means I am taking into account focal ratio (which this article is about), and not taking into account diffraction (which this article is not about).
Here is the problem. You seem to be going out of your way to find instances where the wording of that whole section can be true. Citing diffraction as the basis will make it sound true... but that is not the same as writing a true article. An Encyclopedia should be at least two things---> factual and useful. When I come across the term f-ratio in an article about astronomical telescopes it’s going to take me here. The article then tells me that each numerical value is "halving of the light intensity from the previous stop". Is this factual? No…. it does not have that effect on astronomical objects. Is it useful? No... by definition something that is not factual is not useful. So there is a problem with the wording "telescopes and binoculars may have a fixed aperture, but the same principle holds". What "principle" are we talking about here? By the way this article is written it’s obvious, the definitions that come after the statement are describing "the principle". If we have one instance where a statement is wrong---> the statement is wrong.
Here is a "to-do". When someone is thinking of buying a telescope this Christmas it would be useful if there was an article what would tell them that an 6" f15 Maksutov telescope does not produce an image that is 10 stops dimmer than a 6" f5 Newtonian. Ripping appart this article about f-numbers in photography may not be the most efficient way to do it. Like I said originally we could probably use a more basic article Focal ratio (optics) that gives the basic theoretical principles and then links out to more relevant useful real world aplications like f-numbers. Halfblue 12:18, 7 October 2006 (UTC)
And your f/15 scope is only about 3.1 stops dimmer than f/5, not 10 stops. But, I take your point, if it's the same diameter, it is "dimmer" only in the sense of less total light in the image due to having a smaller field of view, not in the sense of gettig less light per star. At least it's not totally inconsistent with the photographic use, which says the light per area will be less, which it is if averaged over a given size image field when looking at a uniform star field. Dicklyon 15:30, 7 October 2006 (UTC)
And one more thing; you complain: "The article then tells me that each numerical value is "halving of the light intensity from the previous stop". Is this factual? No…." Actually, with a telescope of a fixed image field size, such as a piece of film, or a given size exit pupil coupled to your retina, this is completely true, whether you "stop down" by decreasing the aperture or you choose a longer focal length with the same aperture, if the object field is "uniform" or has stationary statistics. In the one case, you reduce light from a given object field, and in the other you spread the light out so that only a fraction of the star field gets imaged to your image field. Understand this first if you want to add something that clarifies it. Dicklyon 15:36, 7 October 2006 (UTC)
Finally, there's diffraction. See the section on image quality effects. It makes no sense to have an article on f-number without a discussion of diffraction, since that's the first-order thing that limits your image resolution; this is true especially in telescopes. To say theoretically while ignoring diffraction, where diffraction is the dominant effect as it is in determining the image area of point sources, is just nonsense. Dicklyon 15:40, 7 October 2006 (UTC)
Halfblue 12:59, 20 October 2006 (UTC)
Just a thought: stars are not point sources. They are, in fact, very large extended objects. If you were not limited by diffraction, the size of the image of a star would scale with focal length the same way any other image does. The point source model works only because you are below the diffraction limit, so the size of the image of the star does not scale as predicted by geometric optics. The statements in the article about the properties of f-number are true whenever diffraction is negligible, and false otherwise. Point taken, though, about the need for this article to be useful to astronomers as well as photographers and optical engineers. Some reworking of the text may be required to accommodate everyone's needs.-- Srleffler 23:36, 20 October 2006 (UTC)
I quote latest development from a section above:
I argue that that is not an apppropriate response. Take a look at the page. It is mostly just repetition of this f-number article, with a nice new telescope picture. I see no reason it can be incorporated here, avoiding such splintering of content. I will revert it back to a redirect, doing my best to merge any new content here first, unless somebody else objects. At present, it is just a place for halfblue's misconception or misinformation, in which he implicitly assumes that in varying the focal ratio, the only independent variable is focal length, not diameter. Dicklyon 15:10, 20 October 2006 (UTC)
Since someone beat me to reverting it, so I went back and copied the content, corrected it, and put it in a new section in this article. It's a bit redundant, but that's probably OK. Dicklyon 20:28, 20 October 2006 (UTC)
I see the article has been moved to f-number. Here is the reason for the article Focal ratio---
The problem I see with f-number is not a problem with the article its self (although it does have some fixable errors), it is its place in terms of an encyclopedia. To repeat something I said somewhere else--- There is an old Microsoft joke [5] where a software engineer gives an answer that is 100% correct and relatively useless. I get that feeling when I try to link to f-number.
and has a diagram of standard photographic stops opposite that statement. In other words this is an article about the photographic application of "focal ratio" more than the article describing the basic concept of "focal ratio". The lead in paragraph is not a good description of "focal ratio" as it applies to mirrors, radar/radio telescopes, and parabolic sound reflectors, ect. That could mostly be fixed by replacing the word "lens" with the word Objective (optics) but it shows the bias in the article.
I don't know if there is a wiki term for this but "f-number" is getting to be a mega-article. It is trying to describe everything under the sun that has to do with the ratio of stop to focal length and doing it poorly. I was noticing this before with Aperture. If you link Optical telescope - Focal length and f-ratio to f-number the article tells you a few things that are flat out wrong re: telescopes, stellar point sources, and focal ratio such as:
So--- when we send people to f-ratio when they click a link with the question "I wonder what "f-ratio" does to these different types of telescopes?". Coming into the top of this article they are not going to get an immediate answer (and they are going to say "wait a minute... this is about photography"), Then they are going to get a bogus answer, then, if they preserver, scanning down what is getting to be a long article they may find the newly added content on telescopes. Its not the readers fault if they are confused. They have been given an answer that is about 95% correct and (from their laymans point of view) almost totally irrelevant to what they were looking for. I think this is what is being missed and what Wikepedia means when they say Think of the reader. Directly linking them to F-number#Focal ratio in telescopes may work for a wile till someone says "wait a minute.. thats redundant.. lemme clean it up and make that sub title "f-ratio"" and kills all the links (unless there is some redundancy in wiki-linking that I am not aware of).
Some more observations:
That is more than my two cents.. Halfblue 00:14, 21 October 2006 (UTC)
Besides, the stuff you assert is "flat out wrong" is actually correct, as I've explained in various terms. If there's a better way to explain the effects you care about in astronomy, by all means let's add them, as I tried to do in incorporating your focal ratio article as a new section. I see no inherent difficulty in making this article suit the needs of astronomy, microscopy, and photography. Just add the material as suitable. Dicklyon 06:11, 21 October 2006 (UTC)
Halfblue, I did a quick GBS and found a source for your idea: book page. However, the statement that comparing two 8" scopes the image brightness is not at all dependent on focal ratio is restricted by "if the two scopes are used at the same magnification", which changes it completely, into something that is now true. Without the qualifier, one might naturally assume that brightness referred to equal treatments of the focal plane, e.g. same film or same CCD or same eyepiece, in which case you need some other restriction, such as the total optical power in the spot even as its size changes, to make it true. But for either of these, you also need to make it clear that the diameter is being held fixed, and the focal length is what you are varying. Otherwise, claiming brightness independent of focal ratio is just obviously wrong, in general, for any definition of brightness. Dicklyon 06:20, 21 October 2006 (UTC)
Sorry for slow replies.... the ol' Wikipedia hobby has to take a back seat to life sometimes. Each thing gets one article, no matter how many different terms there are that describe it gets you to the heart of the problem with the f-number article. f-number is not the "thing". It is a number resulting from the thing. The thing here is Focal ratio... f over D. f-number is the results of that equation. Articles get renamed or redirected to their more basic form all the time. In this case we got the redirect going the wrong way. Setting up something like focal ratio (astronomy) is not really a fix because focal ratio is not limited to astronomy... or even the optical spectrum. f-number (with a few minor corrections) is a good article on the applications of focal ratio but it would take a lot to make it inclusive of everything about focal ratio (the intro paragraph needs to be totally rewritten for a start). All of that would be tackling the symptom but not the cause... an article with the wrong name. The simple fix is to set up a page for Focal ratio noting that there are many different applications of the principle (which I have already done) and make some minor edits (and maybe even a name change) to f-ratio to note it is about applications of a more basic principle. Halfblue 12:52, 26 October 2006 (UTC)
This section may end up tying together several threads begun in sections above. I see that someone has created a new article on lens speed. At first glance, that might seem to be redundant with this article, but I think it is actually a good idea. As constructed, that article focuses on the concept of "speed" of a lens, and its role in photography. It eschews the technical details, and uses f-number mainly as a benchmark of "speed". That is a nice complement to this article, which has all the technical detail that is so important to optics. f-number is probably too technical for a reader who is an amateur photographer with little interest in optics. I think we should maintain this separation, and ensure that the links between the articles guide photographers with an interest in more technical detail here, and those with no interest in it there.
We may also want to move some material between the two articles. For example, I moved the section with the list of fast lenses to the other article. It is a much better fit there than here, both conceptually and in terms of who is likely to read which article. One option to consider: we might want to deliberately reduce the focus on photography in this article, and make it more aimed at coverage of f-number in general (i.e. encompassing optics, astronomy, photography, etc.), leaving the lens speed article to cover the aspects of particular interest to photographers. I propose this with some doubt, however: it's not clear to me that that is the way to go.-- Srleffler 05:17, 10 November 2006 (UTC)
Hello,
The
Barry Lyndon articles states that Kubrick used a NASA f/0.70 focal ratio. Problem is, I don't have a clue what it means out of technical thing. Could someone give comparison examples from real world? Like how far can it see, how dark can it see, what is the animal that comes closest to this, and so on. Thank you.
David Latapie (
✒ |
@) 08:00, 29 November 2006 (UTC)
I read your definition of F-Number, and I'm sorry to inform you that there is a mistake using the term "square root of 2." The language states "....sequence of the powers of SQRT(2) (or 2^0.5)...." You should correct this to say ...sequence of powers of 2^x with x=0 -> x=n in increments of 1/2 i.e. 2^0, 2^0.5, 2^1.0, 2^1.5, 2^2....Etc. Therefore 2^2 would correspond to an F-Number (or F-Stop) of f/4. 69.44.134.230 16:49, 29 November 2006 (UTC)
I have another mathematical-interpretation question. The article seems to consider that an f-number of f/4 is smaller than f/8. From a mathematical expression however, if f is constant, f/8 is a smaller number than f/4. Unless I am forgetting/misinterpreting something here, I think this is very confusing. It may very well be that in the photographic world this is the way they consider it, but in that case I would mention this confusion in the article. -- Patrickdepinguin 21:13, 26 May 2007 (UTC)
That's right, the f-number 4 is smaller than 8, and the aperture f/4 is larger than the aperture f/8. It's not a confusion, but it means that ordering of f-numbers and apertures need to be made clear. Is there some place in the article that is not clear? Dicklyon 21:49, 26 May 2007 (UTC)
I believe that this is naturally confusing. In photography, one often says "increase" or "decrease" regarding aperture or f/stop. In that case, "increase" means increasing f/#, and so decreasing #. I have never known in photography the use of the words "increase" and "decrease" to correspond with the #, but always its reciprocal. Not that the article is wrong, but it is hard to argue with tradition. Gah4 ( talk) 08:40, 26 September 2011 (UTC)
The new table has no generality, and serves only to illustrate that each full stops cuts the area in half. This is easy enough to see without spending a big table on it, in my opinion. I vote we remove it. Srleffler has already tagged it as questionable. Anybody think it is worth keeping? Dicklyon 05:59, 30 December 2006 (UTC)
I propose we remove this talk fragment. Since the sheet doesn't exist on Wiki, the F/stop page doesn't refer to it, and I'm not planning on adding it again. -- GordonMcKinney ( talk) 01:58, 16 May 2008 (UTC)
F-number is a technical term used to describe the amount of light allowed through a lense system.
The term was originated when photographers used cameras that allowed the photographer to view the scene before exposing film. In order to make the preview as bright and easy to see as possible, the photographer would open the lense aperture as large as possible. Then, before exposing the film, the photographer would close the aperture to account for film speed, motion, and depth of focus. Eventually, cameras opened and reduced the aperature automatically, and one only set the size to which the reduction would be stopped. The aperture starts open and is "stopped down" to the appropriate diameter just prior to exposing the sensor.
Synonyms of f-number have been introduced over the years, many of which survive today. These include: focal ratio, f-ratio, or relative aperture [1].
The focal length of the lense system is used because the intensity of light is concentrated at higher magnifications. Focal length is a simple way to describe the magnification capability of a lense. Many people have an intuitive grasp of this because it can be demonstrated by burning paper with a magnifying glass in direct sunlight. Covering the lense partially prevents the focused beam from burning the paper because the intensity of light is reduced.
Aperture diameter defines to the amount of light allowed through a lense system, and this is related to the length of time the sensor must be exposed to the in-coming light. The relationship between aperture diameter and exposure time is often quoted as lense speed.
Why is focal length important? How can that be incorporated into this article? neffk 15:06, 26 June 2007 (UTC)
I've had time to think and re-read the text of the article and what I tried to contribute. Initially, I was intimidated by Dicklyon, but I think maybe I should have been more bold about discussing the changes I proposed. I see that many of the points were not appropriately addressed. One of the reasons for reverting my lead was an appeal to tradition: "...what's wrong the rather precise way we had it before?" That's against Wikipedia policy, as I understand it; Wikipedia:BRD. Anyway, see the items below below. neffk ( talk) 16:34, 25 June 2009 (UTC)
The relative aperture of a lens in photography is a ratio of two numbers. What numbers are these?
1) is the diameter across the lens opening (the aperture) (Really the effective aperture - which is what can be seen in actual fact by moving the eye from side to side - multi-element lenses will give a different value than the actual diameter)
2) is the focal length of the lens.
The effective aperture (f stop) is supposed to give the same figure for big cameras and small cameras, for big lenses and small ones.
f/11 means the diameter of the lens will divide into its focal length eleven times.
This can also be expressed as 1:11 where 1 is the focal length of the lens. (Lenses are marked say 1:11) This is known as the maximum relative aperture of the lens.
If we write f/11, f means the focal length of the lens and the slash means "divided by".
The relative aperture gives the amount of light reaching the image plane (eg the film) because the inverse square law says that light increases (or decreases)by the square of the distance. Halving the distance to a light source will not double the light it will quadruple it. And increasing the distance by three times will reduce the light by the square of three = 9. Only one-ninth of the illumination will get there.
In a camera, the relative aperture takes this into account. RPSM 16:22, 11 November 2007 (UTC)
Hi, I was looking at other articles, and some people in other places are getting themselves tied up in knots. Here is from article Camera
Various Cameras: An Agfa Brownie, Polaroid Land Camera, and Yashica 35 mm SLR The size of the aperture and the brightness of the scene control the amount of light that enters the camera during a period of time, and the shutter controls the length of time that the light hits the recording surface.
Well - it's the value of the relative aperture innit?
I only have simple math, and I was thinking of explaining f/number in the simplest terms possible to make it available to the youngest and least technical readers (like me).
In optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture[1]) of an optical system expresses the diameter of the entrance pupil in terms of the effective focal length of the lens.
in terms of is no doubt correct, but will it be understood by people with only + - x and ./. ?
I prefer the focal length divided by the effective aperture.
What's the effective focal length?
Is that the actual focal length in use, or what. How can you reduce the explanation to be as simple as possible for even young readers?
You use the term "relative aperture" I think this is the correct term. F stop etc is everyday talk.
This article is fine. I will try to find the other places in wiki where they go into paroxysms of mathematics without starting from basic principles. Best RPSM 16:47, 11 November 2007 (UTC)
Did you do the article on '''Aperture''' as well?
That looks ok.
I'm just into very simple explanations for beginners without too much math. It doesn't have to be like that on Wiki.
At some point the relationship between basic definitions (focal length, f/stop, etc) and how the math works forces non-mathematicians to say: now I have to get the hang of this in mathematical terms.
Of course light captured by a lens is one thing, and light received at the image plane another.
Are we going to go into foot-lamberts and lux as well?
And what is the math for exposure meters and such? RPSM 17:04, 11 November 2007 (UT
f-number is defined as the ratio of focal length to the diameter of the entrance pupil.
This is what you said in chat above: I like that better.
Of course, when you look at a lens it has on it either F/4.5 or else 1:4.5 as well as the focal length expressed as f= 7" or f=5cm.
I just thought that beginners could relate theory to practice by showing that these are mathematical expressions that give a key to the theory. ¨¨¨¨ —Preceding unsigned comment added by RPSM ( talk • contribs) 18:09, 11 November 2007 (UTC)
One origin of the word stop is from Waterhouse stops, which, as you know, are slips of metal with a hole in. Let's say you are only using one stop all the time, you stick in the stop and you don't have to read any scale or anything. And you can directly measure the diameter on a rule. They are used with lenses for large field cameras or studio cameras. In that case, you did not have to go round to the front of the camera and peer at the aperture scale.
The other type - normal today - is the iris diaphragm. (diaphragm alone is used for a number of things in other fields) RPSM 18:15, 11 November 2007 (UTC)
We don't have a simple explanation of focal length. I always say it is the distance from the optical centre of the lens to the image plane when the camera is focused on infinity. Simpler: When the camera (lens) is focused on a distant object - a range of mountains or the sun, the rays of light entering the lens are parallel, or very nearly so for our purposes. Then the distance from the optical centre of the lens to the film (image plane) is the same as the focal length of the lens. http://www.paragon-press.com/lens/lenchart.htm is a simple explanation. However I like the full-blown technical explanation as well, and I want relative focal length back somewhere. I did a photography exam once and one question was: What is the difference between relative aperture and effective aperture? and it wasn't in the books we had. Now I get it after seeing it here. Thanks. RPSM 13:58, 12 November 2007 (UTC)
Sorry, effective focal length I'm learning all the time. RPSM 14:45, 13 November 2007 (UTC)
First two references are for opticians (eye doctors), third one is from Slovenia: http://www.nfos.org/degree/opt11/module_13a.html http://www.nfos.org/degree/opt11/module_13b.html "So now we come to the second most used formula in optical theory -- Prentice's rule. This formula gives us an approximation of the amount of prism present at any given point on a lens based on the dioptric power of the lens and the distance that the point is from the optical center of the lens." [I think they mean the optical axis (?)] Next reference is better: http://lrv.fri.uni-lj.si/~peterp/publications/prl06.pdf "The optical center within the camera represents the point in which all the light rays that form the image intersect if the lens is approximated with a single thin lens." It looks like these people were having a hard time trying to find the optical centre - Is that what you mean by saying it is undefined? RPSM 14:50, 13 November 2007 (UTC)
Thanks RPSM 08:01, 14 November 2007 (UTC)
I've seen image-space f-number = effective focal length / entrance pupil diameter and paraxial working f/# = 1/ (2 tan U') where U' is the angle between the entering marginal ray and the exiting marginal ray. Also, working f/# = 1/(2 sin U'). Should this article cover these topics or should they get their own pages? 155.212.242.34 19:21, 3 December 2007 (UTC)
The article says f/number measures lens speed. It seems to me that effective lens speed will have more to do with the entrance pupil area times the spherical angle of view divided by the area of the sensor. That is, how much light comes in divided by the area it is projected against. Furthermore, in theory I could imagine a lens with a huge entrance pupil and a narrow angle of view but a tiny exit pupil and a tiny back focal length. Obviously within one lens, changing the f/number changes the amount of light accordingly, but between lenses, what does it really measure? 155.212.242.34 ( talk) 17:19, 27 December 2007 (UTC)
Here are a couple of good books about all this. Lee speaks of both the entrance-pupil-based and exit-pupil-based approach to f-number, and the fact the pupil magnification may make them not equal; for object at infinity, they will be equal, but otherwise it gets complicated. Also, note that the other book uses positive magnification convention. Dicklyon ( talk) 06:56, 25 January 2008 (UTC)
Now that we've got m negative again, in agreement with the cited source, and "working" f-number being N*(1-m), maybe we should reconsider. When I search books for "working f number", I get pretty much nothing. But with "effective f number" I get quite a lot of hits, and at first glance it appears that more of them use the photography convention of unsigned m, rather than the optics convention of algebraic m. The forumula with 1+m (or 1+M) is common in these books. Should we consider re-wording, and at least give the two conventions equal weight? Dicklyon ( talk) 01:25, 25 January 2008 (UTC)
I vote for the photographically conventional positive / unsigned magnification. This case is explicitly mentioned in the magnification article too. So I would suggest to change all photographically related occurrences of M and especially this odd N*(1-M). Alex.altmaster ( talk) 17:05, 29 January 2008 (UTC)
There is a discussion on a change in typography for f/ happening at Template talk:F/. Please join in if you care about f/ vs. f/.-- Srleffler ( talk) 05:39, 3 August 2008 (UTC)
This sentence from "Effects on Image Quality" doesn't make sense to me. Is it missing a phrase, or am I just confused? "Therefore, reduced-depth-of-field effects, like those shown here, will require smaller f-numbers (and thus more complex optics) than do larger format cameras." Billgordon1099 ( talk) 16:55, 2 September 2008 (UTC)
maybe a footnote on 3.3 would be in place? It should really be 3.2, since 2^((10/3)/2)=3.1748, but probably for some historical reasons the 3.3 from the half scale was inherited.
There is even a note on 3.2, but it might be confusing that 3.2 is not to be found on the scale.
Cheers,
Bo —Preceding unsigned comment added by Babbletower ( talk • contribs) 22:17, 20 September 2008 (UTC)
I don't know if there is a historical significance to f3.3 but in the half-stop table, 2^[(7/2)(0.5)] ≈ 3.36 = 3.4 by round off rules. —Preceding unsigned comment added by 50.46.174.62 ( talk) 16:48, 23 April 2011 (UTC)
I had some minor issues with the current intro text; eg. the confusing use of "effective" aperture ... and the unintentional suggestion that "f-number" was the preferred term for relative aperture in the scientific field of optics ... minor stuff, but I drafted a "new version" for your approval ;)
It might also be helpful/illustrative to point out the the focal ratio on many zoom lenses includes two values for maximum aperture; eg "1:3.5-5.6", the first referring to max aperture at shortest focal length (wide end), the second at longest (full zoom). AFAIK, the maximum physical aperture itself does not change relative to focal length, but the relative aperture does. -- Redbobblehat ( talk) 06:31, 18 February 2009 (UTC)
There seems to be disagreement whether the focal length of the human eye is 17mm or 22mm. Look, for example http://hypertextbook.com/facts/2002/JuliaKhutoretskaya.shtml .
Is there anyone who can explain the discrepancy in detail?
I think it would be appropriate to add (or link to) some information about the variable light sensitivity of the eye. Seems like this kind of information should be available somewhere in Wikipedia, but I didn't find it. neffk ( talk) 13:04, 26 June 2009 (UTC)
This section says the human eye in the dark opens to about f/2.1. Is this true? That would mean that certain lenses would actually be brighter than the human eye ( Sigma 50mm f/1.4, Nikon 35mm f/1.8G, Canon 50mm f/1.4) and that doesn't seem right. -- The High Fin Sperm Whale ( Talk • Contribs) 05:14, 20 December 2009 (UTC)
This article needs some discussion of the manufacturing precision/glass quality required to achieve low f numbers. When I was getting into photography, I got the sense that low f numbers indicated high-quality lenses but wondered why they didn't just allow the aperture to open up further. Similarly, you see old lens designs listed as being an "f/16 landscape lens" and that eventually lens design allowed for f/2.8 lenses and faster. There's obviously potential issues with vignetting, but beyond that, I asked myself "Why can't you take an f/16 lens and make the aperture bigger?"
The answer, as I understand it, is threefold. First is vignetting: Making a faster version of some designs would lead to negative thickness at the edges of elements. Second is the lens design itself, that fundamentally aberrations would start to dominate if you opened a lens up, so while you could make a fast version of an old slow lens, even ideally constructed it would produce very soft images, even at the center of the image. Third is manufacturing quality, that fast lenses are more sensitive to imperfections in surfaces and alignment, since for an entire pencil of rays to come to diffraction-limited focus, the entirety of every surface that pencil crosses must be the right shape to within wavelength tolerances across those areas.
Do those three sound right? If so, let's add some mention of it. —Ben FrantzDale ( talk) 12:48, 7 January 2011 (UTC)
The diagram showing how two lenses with different focal lengths but the same aperture setting, and different aperture diameters will produce the same illuminance in the focal plane appears to be incorrect.
I know that the caption states that "Focal lengths and aperture sizes are not to scale", but if it is drawn to scale, it looks a little different. See here for a diagram drawn to scale, where it shows that the lines meeting the aperture diameter actually meet at a point on the focal plane. Martybugs ( talk) 12:57, 8 January 2011 (UTC)
In this diff, User:Glockenklang1 prefers this new lead section:
The ratio of focal length and aperture of an imaging system is dimensionless and defines the F-number [1]
F is identical to the inverse of the relative aperture( [2]) and is referred to also as focal ratio, f-ratio, f-stop, "f/#".
More cautiously the aperture has to be a linear measure of the entrance pupil of rays forming the image, thus the effective aperture. The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.
The inverse of F-number is often used in astronomy.
The Iluminance of the image goes inversely with the square of the F-number
A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 135 mm focal length lens with a setting of f/4 will have a pupil diameter of about 33.8 mm. The 135 mm lens' f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.
Other types of optical system, such as telescopes and binoculars may have a fixed aperture, but the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image).
to this old stable version:
In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture [3]) of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter. It is a dimensionless number that is a quantitative measure of lens speed, an important concept in photography.
The f-number (f/#) is often notated as and is given by
where is the focal length, and is the diameter of the entrance pupil. By convention, "f/#" is treated as a single symbol, and specific values of f/# are written by replacing the number sign with the value. For example, if the focal length is 16 times the pupil diameter, the f-number is f/16, or . The greater the f-number, the less light per unit area reaches the image plane of the system; the amount of light transmitted to the film (or sensor) decreases with the f-number squared. Doubling the f-number increases the necessary exposure time by a factor of four.
The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.
A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 135 mm focal length lens with a setting of f/4 will have a pupil diameter of about 33.8 mm. The 135 mm lens' f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.
Other types of optical system, such as telescopes and binoculars may have a fixed aperture, but the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image).
I think this is too much to change without talking about it. Definition, calling it "F", lead image caption, initial section titles, lots of text, etc. Some of these changes may be OK, but it's not clear what problem he's trying to solve. Opinions?
Dicklyon (
talk) 01:03, 6 February 2011 (UTC)
The ratio of focal length and aperture of an imaging system is dimensionless and defines the f-number
N is referred to also as focal ratio, f-ratio, f-stop. Optical Lenses are very often denoted by "f/N". Example: a lens with f=400mm D=100mm the formula gives N=4 and would be denoted by f/4.
More cautiously the aperture has to be a linear measure of the entrance pupil of rays forming the image, thus the effective aperture. The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.
The Iluminance of the image goes inversely with the square of the f-number
A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. The same lens set to f/8 will have D=12.5mm and supply only 1/4 of the amount of light for exposure. Another lens with f=135 mm setting of f/4 will have a pupil diameter of about 33.8 mm. The 135 mm lens' f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.
Other types of optical system, such as telescopes and binoculars may have a fixed aperture, but the same principle holds: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image). -- Glockenklang1 ( talk) 21:24, 11 February 2011 (UTC)
I agree with Jeff. I still don't see what problem is being addressed by these proposed changes. The lead sentence gets away from normal style, and mentions simple tangents like "dimensionless" before mentioning the topic and the alternative terms for it, which conventionally comes first. The other names shouldn't be indirected through N, and we don't need an example in the lead paragraph. It's not clear where the "caution" thing is coming from, as there's no danger apparent; the entrance pupil is already mentioned, as opposed to the more ambiguous term "aperture" (which is never intended to mean the aperture stop's diameter, I'm pretty sure). Dicklyon ( talk) 06:34, 12 February 2011 (UTC)
The current formula for Working f-number, though correct, is uncommon in photography, where inversion of the image is typically ignored, and magnification is always taken as positive (e.g., Ray 2002). I think this section would be more clear if we mentioned the photographic convention and included the formula as it is given in most books on photographic optics, where it’s often called the effective f-number:
Many books on photographic optics, including Ray (2002), distinguish between the effective f-number and the marked f-number (the one given by the formula at the beginning of this article and usually given for camera lenses). Perhaps we should mention this as well. JeffConrad ( talk) 10:15, 13 February 2011 (UTC)
Umm . . . wudja believe
JeffConrad ( talk) 14:19, 13 February 2011 (UTC)
My ongoing additions and clarifications to this article in regard to the vendor specific usage of f-number scales have been reverted by Dicklyon two times. My lastest in-edit version can be inspected here:
http://en.wikipedia.org/?title=F-number&diff=438918941&oldid=438902195
Dick, you wrote that my additions were unsourced and original research and likely untrue. While I cannot claim to be free of errors, the information I gave was carefully compiled and cross-checked from various sources over the years, and in those cases where I found diverting infomation I either omitted the info or added a ? to indicate this. The information on Minolta, Konica Minolta, and Sony A-mount cameras is definitely correct, as I have verified the information with my own large set of cameras and lenses (1985-2011). Even the values outside the range of currently existing lenses have been verified by inspecting the cameras on protocol level and using "fake" adapters to emulate lenses with properties not yet commercially available. Also, some of the camera firmware images have been reverse-engineered and corresponding display values be found in the firmware. I even gave quite a number of references with detailed background information (including proof photos), however, I did not want to clutter the article with references, that's why I gave most of them in the form of HTML source code comments < ! - - - - > not via < ref > tag. Maybe you missed this? The f-numbers displayed by Minolta, Konica Minolta and Sony A-mount cameras have been consistent over all the years, with basically just one exception: While Minolta bodies never reported an aperture of 1.8 (in 1/2EV or 1/3EV steps), Sony cameras started to report both 1.7 and 1.8 depending on the lens used - this is down to internal rounding. Otherwise, all camera bodies report the same values from 1.0 to 64, and the Konica Minolta and Sony cameras also seem to support values downto 90 (and there are some reports of values downto 128, but I did not included this information, because I could not verify it). The Minolta 1/4EV f-number scale is complete and correct as well - there have been in total three Minolta devices supporting the display of apertures in 1/4EV granularity, and I have verified the displayed values on each of them. There seems to be more inconsistency in the f-number scales used by Canon and Pentax, at least older cameras (such as the T90) reported different values than EOS cameras do, and I also found some different information in Canon service manuals regarding some values. Since I do not use Canon and Pentax cameras myself I have to rely on third-party information here (but several user reports have been used to verify the information as much as possible). I am quite sure that even with some inconsistencies here, it is possible to draw a timeline somewhere when these vendors changed their scales. I do think, that this information is encyclopedic and relevant and useful to the article. The article in the current form teaches typical usages of 1/2EV and 1/3EV f-number scales which in fact do not reflect the typical usage of f-numbers of some of the largest camera manufacturers today (and in the past twenty years or so). That's what I would call "likely untrue" (and it is not sourced in any way). I do think that adding this information to the tables (and not to the text) is the best way to represent it, because readers can then easily compare the f-numbers scales this way. Do you have a better idea? Ideally, I would like to see this article list and compare the f-number scales used by *any* camera manufacturer since ca. 1985 (when cameras started to display f-numbers in 1/2 or 1/3EV steps). In some cases, it might be necessary to list more than one scale per vendor, but it wouldn't harm, since the number of vendors is limited. -- 88.77.195.147 ( talk) 18:43, 11 July 2011 (UTC)
In ZEMAX, there are three f/#s listed in the System Data report: Image Space f/#, Paraxial Working f/#, and Working f/#. I'm trying to understand what they mean and the ZEMAX documentation is a bit thin.
Could someone comment on these? I haven't found a reference anywhere that gives good optical-engineering definitions. —Ben FrantzDale ( talk) 17:05, 4 August 2011 (UTC)
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