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Is a F-stop basically a camera's spatial filter? That is it is an iris positioned at the focal point between the lens system of the camera? Waxsin ( talk) 16:10, 2 February 2016 (UTC)
It took me a while to wrap my head around the definition of lens brightness in this article, but I think what is in the article now is correct. The brightness of the projected image is pretty straightforward: the illuminance on the image sensor, or luminous flux per unit area. The brightness of the scene is the trickier one - I'm pretty sure it is luminance, or luminous intensity per unit area of light travelling in a particular direction and passing through a given area. What area? the front of the lens. What direction? towards the front of the lens and within the lens's field of view. Scene brightness when described this way is largely invariant of the size of the front of the lens, because of the m^-2 in the unit, as it should be: make the front of the lens larger and more light from the scene will fall on the front of the lens, but the scene looks just as bright to the lens, so your description of scene brightness should be normalized against the area of the front of the lens. Illuminance and luminance are measured in different units, but that is o.k. Lens brightness in f-stops or t-stops is really a description of the sensor illuminance to scene luminance quotient, which would have a unit of steradians. Please speak up here if you aren't in agreement. Balazer ( talk) 21:05, 15 January 2013 (UTC)
I have a question about the fourth paragraph in the "Notation" section. The first three sentences are very clear - explaining how the 200mm lens receives four times as much light. In the next sentence, I believe the focal length numbers need to be reversed, since the 100mm lens is wider than the 200mm lens, and would therefore be the one that covers four times the area. As it's written, it sounds like the 200mm lens produces 16 times the illuminance. BigslyE5 ( talk) 15:09, 26 July 2014 (UTC)
And that is why or focal length divided by f-stop equals Diameter. 19dreiundachtzig ( talk) 00:35, 18 December 2020 (UTC)
I'm in favor of using entrance pupil instead of aperture in the definition of f-number, because entrance pupil is more precisely defined. There are multiple types of apertures, which are not all the same. The aperture formed by a lens's diaphragm, for example, is usually not the same as the entrance pupil and not what the f-number is defined in terms of. But we must recognize that in common photography speak, people say aperture when they really mean entrance pupil. So I wrote in the first sentence of the Notation section that the entrance pupil is often called the aperture. I'd be fine to move that statement up into the definition also, but I think we should maintain a distinction between aperture and entrance pupil, and not turn them into synonyms. Lens design textbooks are pretty consistent about using entrance pupil or clear aperture, and never just aperture. Balazer ( talk) 04:30, 19 January 2013 (UTC)
While as written it seems technically correct, it seems to me that actual photographers do it differently.
Well, maybe I have never heard a photographer say "increase the f-stop" but always "increase the aperture." Now, since as written it is actually a fraction with the numerical part in the denominator, is it wrong to say that, for example, f/8 is larger than f/11? (Is the f/number 8 or 1/8?)
Continuing, photographers usually talk about shutter speed, rarely shutter time, and it is more usual to label the shutter dial with the reciprocal of the time in seconds. That is, 125(/second) and not 1/125(seconds). (In EE terms, the inverse of period is frequency, but that doesn't seem quite right here.) (When the time is longer than 1s, it might be that calling it a time instead of speed is not unusual, but even then speed might be used.)
Similarly, resolution should be an inverse length (spatial frequency) not a distance (dot pitch), such that "high resolution" has the right meaning. Gah4 ( talk) 09:12, 4 March 2013 (UTC)
As I remember it, the aperture terminology was "open up"(f/11 to f/8), "stop down"(f/8 to f/11), "widen the aperture"(f/11 to f/8), "narrow the aperture"(f/8 to f/11) ... but "increase the aperture"(what are you talking about? stop trying to confuse us.)?
Depending on if you think of the aperture as being the hole which allows the light to pass, or the circular structure that restricts variable amounts of light, it could go either way. While the technical definition may go one way or the other, the phrase "increase the aperture" is ambiguous and non-specific as to the many-varied cogency and oft-reversible thinking of the absent-minded photographer. It could mean increase the light being stopped, or increase the light allowed through. I suggest using a different phrase. JimsMaher ( talk) 14:35, 7 March 2013 (UTC)
When teaching this material (which I do for college multimedia production classes), aperture (or the effective term "entrance pupil" used here) and f-stop numbers need to be consistently spoken of in the same breath to avoid the confusion we're talking about. We put our hands in a circle, then make it larger or smaller, saying, "Here's f/2... and here's f/11..." constantly reiterating that the larger aperture has a smaller f-stop number because the f-stop is a ratio. It is my feeling that the same approach should be used in this article as well - and for the most part that is the case. I agree that "increase the aperture" clearly means making the hole larger. During instruction it is common to use redundancy in a single reference - referring to the aperture being larger or smaller, more opened or closed, increased or decreased - to give students a sense that there are multiple ways to say the same thing. BigslyE5 ( talk) 14:55, 26 July 2014 (UTC)
I would like to have more detail on the importance in the introduction? How about replacement of "It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography" by "The f-number is a quantitative measure of lens speed, and contributes to the Exposure value", which is a very practical photographic property.
Glockenklang1 ( talk) 10:06, 14 July 2013 (UTC)
Nanette L. Salvaggio Basic Photographic Materials and Processes
Sidney F. Ray Applied Photographic Optics: Lenses and Optical Systems for Photography
Nanette Salvaggio makes the mistake of saying "effective aperture" instead of "relative aperture" - a common mistake as Sidney F. Ray points out.
RPSM ( talk) 08:44, 12 January 2014 (UTC)
This is more of a question that comes out of ignorance. When I calculate the half f-stops series using the formula published in the article the resulting series rounded would yield 3.4 instead of 3.3, 5.7 instead of 5.6, 23 instead of 22 etc. I tried to find on google why the rounding seems to follow a rather arbitrary sometimes up sometimes down rule, but I was unable to shed light on this. My obsessive nature would like to know why. — Preceding unsigned comment added by 50.138.183.186 ( talk) 17:33, 9 April 2014 (UTC)
Sorry to come back to such an old topic.
The table below
Standard full-stop f-number scale is not realistic. AFAIK, the fastest lens was the
Carl Zeiss Planar 50mm f/0.7. Hence, we should drop ƒ 0.5. Although I have seen lenses with ƒ 90, they are practically useless due to extreme diffraction (images are very soft). Don't know whether a lens with ƒ 128 exists at all. ƒ 256 is already a
pinhole. Given. But: What puzzles me (like IP 50.138.183.186) is the rounding issue. I understand that there is limited space on a lens to print numbers with more than two significant digits. But why 5.6 instead of 5.7, 22 instead of 23, and 90 instead of 91? I failed to find any reference. Any ideas?
Alfie
↑↓
© 00:23, 1 December 2022 (UTC)
In the "Effects on image sharpness" section, the article makes two contradictory points:
These two points are contradictory because
Assuming that we are using the large-format and the small-format cameras to take the same picture (i.e., same angle of view and subject distance), according to the first point, the entrance pupil diameter (i.e., effective aperture) required to maintain a particular depth of field would remain unchanged. Sure, the small-format camera lens would have a smaller f-number (to match the correspondingly smaller focal length needed to maintain the angle of view), but the aperture size (entrance pupil diameter) would be the same. To clarify, the small-format camera would NOT require a larger aperture than the large-format camera.
I suppose that the second point can also be interpreted in an alternate way: instead of holding the angle-of-view constant (to get the same photo), we can instead hold the focal length constant between the the large- and small-format cameras. However, this comparison between cameras would be unusual since the two cameras wouldn't be taking the same picture: the photo on the small-format camera would appear more zoomed in. Still, even if this alternate interpretation is intended, point 2 does not appear to be true: when the focal lengths and aperture sizes are identical between the large- and small-format cameras, we are more-or-less using the same lens for both cameras, so for the same subject distance, the circles-of-confusion would be identical in size.
If anything, assuming that both the large- and small-format cameras have the same number of pixels, the pixels on the small-format camera would be more closely spaced; as a result, the circle-of-confusion would cover more pixels in the small-format camera, making the blur more obvious there.
Can someone help me double-check this? In any case, the second point (that small-format cameras would require larger apertures to maintain the same depth-of-field) is not obvious; if it stays unchanged, I think it should be supported by a citation.
Best regards, -Jimmy C. Chau 128.197.53.42 ( talk) 22:56, 16 July 2015 (UTC)
I have removed this incorrect parenthetical clause from the article. Jcchau ( talk) 02:58, 24 July 2015 (UTC)
N=f/D is unreliable as a definition of f-number and most often only an approximation. For instance N is not equal to f/D in the typically shown thin lens diagram, as explained by Kingslake: "It is a common error to suppose that the ratio D/2f is actually equal to tan(theta), and not sin(theta)" [1]. So assuming f/stop = f/D maybe fine as a rule of thumb in the field, but fails when put to the test. For instance it may induce people to assume that N can be made arbitrarily small simply by making lens diameter arbitrarily large. Or that one could get an arbitrarily low equivalent f/stop by mounting a reference lens on an arbitrarily small sensor. Both of which are physically impossible for a practical photographic lens.
N is on the other hand always equal to 1/[2sin(theta)] in air for a practical photographic lens that produces reasonable images, per Kingslake and others [2]. Knowing the precise definition of N allows a photographer to decode advanced optics formulas and to answer questions like "what's the smallest f-number possible for a reasonable photographic lens?" (Obvious answer: minimum N = 0.5 in air). That's not possible with the article as it stands.
Therefore I would suggest specifying in the initial definition that N = f/D is a useful approximation that works fine most of the time in the field. But I would also add a section with the precise definition of f-number N = 1/[2nsin(theta')], with n the index of refraction of the medium between the lens and the image plane. That's air in interchangeable lens systems, so n=1. It should then be apparent that N = 1/2NA exactly, with NA = Numerical Aperture.
Can page admins make the relative changes? Jack Hogan ( talk) 16:54, 3 March 2016 (UTC)
Numerical_aperture#Numerical_aperture_versus_f-number explains tan(θ) vs. sin(θ). Gah4 ( talk) 16:18, 13 September 2018 (UTC)
References
On a camera, the aperture setting is usually adjusted in discrete steps, known as f-stops. This was usually true from the rangefinder days through the manual focus SLR days. Though there are detents at full stops, the aperture changes continuously. The later electronic SLRs, and continuing into digital SLRs, normally allow steps of 1/2 or 1/3 stop. The settings are electronic, and not continuous. Seems like we should at least take out the usually. Gah4 ( talk) 07:55, 20 July 2016 (UTC)
Thanks, page useful. Will read in detail, but at this point just trying to help daughter into using the loaned camera. THe split image showing effect of f-number - great. Below a just as useful image with caption. "Shallow focus with a wide open lens" - if this just said (high f-number) or (low f-number) it would be great, much easier to grasp by those reading the page to learn. I totally, understand that if I already knew this all, read the page thoroughly, it would be redundant infromation, but that's why I'm reading it. As a wonderfully informative page, it would be even easier with this consistency of comment in the caption as well. — Preceding unsigned comment added by 122.60.105.241 ( talk) 23:53, 25 August 2018 (UTC)
Can someone rewrite this section? I understand many aspects of photography, but I did not understand this section. Could someone please write it in layperson language, clarify what is being said, and expand it a bit to make it more useful? because it looks like it has the potential of being a very useful section! Misty MH ( talk) 03:43, 10 November 2018 (UTC)
Regarding the correction for objects not at infinity, with TTL (through the lens) metering systems, this will automatically be corrected. Should this be mentioned? Gah4 ( talk) 04:33, 10 November 2018 (UTC)
The article says Most old cameras had a continuously variable aperture scale. I suppose this depends on how old you mean by old. Many box cameras from the early 1900's have a sliding metal tab, or rotating metal disk, with holes in it. The continuous aperture might have been from about the 1930's or so. Many cheaper cameras only offer one aperture setting. With electronic aperture control, the continuous scale went away. Gah4 ( talk) 04:46, 27 March 2019 (UTC)
@ Rkinch: Regarding this edit, please provide a page reference in the book cited, and a quote. I do not believe the new definition you have put in the article is correct.-- Srleffler ( talk) 03:48, 12 August 2019 (UTC)
...energy collected from a small area of the object is directly proportional to the area of the clear aperture, or entrance pupil, of the lens. At the image, the illumination (power per unit area) is inversely proportional to the image area over which this object is spread. Now the aperture area is proportional to the square of the pupil diameter, and the image area is proportional to the square of the image distances, or focal length (f). Thus, the square of the ratio of these two dimensions is a measure of the relative illumination produced in the image.
The ratio of the focal length to the clear aperture of a lens system is called the relative aperture, f-number, or "speed" of the system, and...
A recent change removes photography as the use of f/number. As well as I know, optics other than photography, uses Numerical Aperture, symbolized as N.A. In most cases, N.A. is twice the denominator of the f/number. (That is, if you think of it as a fraction.) Is there a WP:RS that shows it widely used in optics, other than photography, or I suppose when photographic lenses are used in non-photographic applications. Gah4 ( talk) 01:22, 9 August 2020 (UTC)
More specifically, this place:
″A 100 mm focal length f/4 lens has an entrance pupil diameter of 25 mm. A 100 mm focal length f/2 lens has an entrance pupil diameter of 50 mm. Since the area varies as the square of the pupil diameter, the amount of light admitted by the f/2 lens is four times that of the f/4 lens. To obtain the same photographic exposure, the exposure time must be reduced by a factor of four.
A 200 mm focal length f/4 lens has an entrance pupil diameter of 50 mm. The 200 mm lens's entrance pupil has four times the area of the 100 mm lens's entrance pupil, and thus collects four times as much light from each object in the lens's field of view. But compared to the 100 mm lens, the 200 mm lens projects an image of each object twice as high and twice as wide, covering four times the area, and so both lens produce the same illuminance at the focal plane when imaging a scene of a given luminance.″
The excerpt's second paragraph doesn't mention what f-number of the compared 100mm focal length lens is? Is it from the previous example of the first paragraph? If yes, is it f/2 or f/4 100 mm lens? Be more specific and detailed about such details, please.
"The 200 mm lens's entrance pupil has four times the area of the 100 mm lens's entrance pupil, and thus collects four times as much light from each object in the lens's field of view" - why? Expand on that with a couple of words more (by including relevant information in the article, not here).
Also, going back to the same example: "Since the area varies as the square of the pupil diameter" - why? This step occurred abruptly after the preceding one without any transition explaining how the square was come up with. Scrutinizer798 ( talk) 01:13, 23 September 2020 (UTC) Scrutinizer798 ( talk) 01:03, 23 September 2020 (UTC)
The article currently reads, "Modern electronically controlled interchangeable lenses, such as those used for SLR cameras, have f-stops specified internally in 1⁄8-stop increments, so the cameras' 1⁄3-stop settings are approximated by the nearest 1⁄8-stop setting in the lens." I'd really love to see a citation for this! It seems really unlikely, given that aperture is often controlled by a solenoid on cheap cameras, and a stepper motor on more expensive ones. 71.63.160.210 ( talk) 09:01, 27 December 2021 (UTC)
(I know essentially nothing about photography, and I think this article does an astonishingly good job of explaining its complex topic.)
In a couple of places, it is pointed out that certain assumptions are only strictly valid when the subject of the photograph is infinitely far away, and therefore those assumptions must be modified when the subject is close to the lens. But how close is "close"? In other words, at what distance from the lens does it become necessary to compensate for the closeness of the subject? I imagine it might be proportional to certain characteristics of each lens. TooManyFingers ( talk) 16:14, 19 September 2023 (UTC)
This article is rated B-class on Wikipedia's
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Is a F-stop basically a camera's spatial filter? That is it is an iris positioned at the focal point between the lens system of the camera? Waxsin ( talk) 16:10, 2 February 2016 (UTC)
It took me a while to wrap my head around the definition of lens brightness in this article, but I think what is in the article now is correct. The brightness of the projected image is pretty straightforward: the illuminance on the image sensor, or luminous flux per unit area. The brightness of the scene is the trickier one - I'm pretty sure it is luminance, or luminous intensity per unit area of light travelling in a particular direction and passing through a given area. What area? the front of the lens. What direction? towards the front of the lens and within the lens's field of view. Scene brightness when described this way is largely invariant of the size of the front of the lens, because of the m^-2 in the unit, as it should be: make the front of the lens larger and more light from the scene will fall on the front of the lens, but the scene looks just as bright to the lens, so your description of scene brightness should be normalized against the area of the front of the lens. Illuminance and luminance are measured in different units, but that is o.k. Lens brightness in f-stops or t-stops is really a description of the sensor illuminance to scene luminance quotient, which would have a unit of steradians. Please speak up here if you aren't in agreement. Balazer ( talk) 21:05, 15 January 2013 (UTC)
I have a question about the fourth paragraph in the "Notation" section. The first three sentences are very clear - explaining how the 200mm lens receives four times as much light. In the next sentence, I believe the focal length numbers need to be reversed, since the 100mm lens is wider than the 200mm lens, and would therefore be the one that covers four times the area. As it's written, it sounds like the 200mm lens produces 16 times the illuminance. BigslyE5 ( talk) 15:09, 26 July 2014 (UTC)
And that is why or focal length divided by f-stop equals Diameter. 19dreiundachtzig ( talk) 00:35, 18 December 2020 (UTC)
I'm in favor of using entrance pupil instead of aperture in the definition of f-number, because entrance pupil is more precisely defined. There are multiple types of apertures, which are not all the same. The aperture formed by a lens's diaphragm, for example, is usually not the same as the entrance pupil and not what the f-number is defined in terms of. But we must recognize that in common photography speak, people say aperture when they really mean entrance pupil. So I wrote in the first sentence of the Notation section that the entrance pupil is often called the aperture. I'd be fine to move that statement up into the definition also, but I think we should maintain a distinction between aperture and entrance pupil, and not turn them into synonyms. Lens design textbooks are pretty consistent about using entrance pupil or clear aperture, and never just aperture. Balazer ( talk) 04:30, 19 January 2013 (UTC)
While as written it seems technically correct, it seems to me that actual photographers do it differently.
Well, maybe I have never heard a photographer say "increase the f-stop" but always "increase the aperture." Now, since as written it is actually a fraction with the numerical part in the denominator, is it wrong to say that, for example, f/8 is larger than f/11? (Is the f/number 8 or 1/8?)
Continuing, photographers usually talk about shutter speed, rarely shutter time, and it is more usual to label the shutter dial with the reciprocal of the time in seconds. That is, 125(/second) and not 1/125(seconds). (In EE terms, the inverse of period is frequency, but that doesn't seem quite right here.) (When the time is longer than 1s, it might be that calling it a time instead of speed is not unusual, but even then speed might be used.)
Similarly, resolution should be an inverse length (spatial frequency) not a distance (dot pitch), such that "high resolution" has the right meaning. Gah4 ( talk) 09:12, 4 March 2013 (UTC)
As I remember it, the aperture terminology was "open up"(f/11 to f/8), "stop down"(f/8 to f/11), "widen the aperture"(f/11 to f/8), "narrow the aperture"(f/8 to f/11) ... but "increase the aperture"(what are you talking about? stop trying to confuse us.)?
Depending on if you think of the aperture as being the hole which allows the light to pass, or the circular structure that restricts variable amounts of light, it could go either way. While the technical definition may go one way or the other, the phrase "increase the aperture" is ambiguous and non-specific as to the many-varied cogency and oft-reversible thinking of the absent-minded photographer. It could mean increase the light being stopped, or increase the light allowed through. I suggest using a different phrase. JimsMaher ( talk) 14:35, 7 March 2013 (UTC)
When teaching this material (which I do for college multimedia production classes), aperture (or the effective term "entrance pupil" used here) and f-stop numbers need to be consistently spoken of in the same breath to avoid the confusion we're talking about. We put our hands in a circle, then make it larger or smaller, saying, "Here's f/2... and here's f/11..." constantly reiterating that the larger aperture has a smaller f-stop number because the f-stop is a ratio. It is my feeling that the same approach should be used in this article as well - and for the most part that is the case. I agree that "increase the aperture" clearly means making the hole larger. During instruction it is common to use redundancy in a single reference - referring to the aperture being larger or smaller, more opened or closed, increased or decreased - to give students a sense that there are multiple ways to say the same thing. BigslyE5 ( talk) 14:55, 26 July 2014 (UTC)
I would like to have more detail on the importance in the introduction? How about replacement of "It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography" by "The f-number is a quantitative measure of lens speed, and contributes to the Exposure value", which is a very practical photographic property.
Glockenklang1 ( talk) 10:06, 14 July 2013 (UTC)
Nanette L. Salvaggio Basic Photographic Materials and Processes
Sidney F. Ray Applied Photographic Optics: Lenses and Optical Systems for Photography
Nanette Salvaggio makes the mistake of saying "effective aperture" instead of "relative aperture" - a common mistake as Sidney F. Ray points out.
RPSM ( talk) 08:44, 12 January 2014 (UTC)
This is more of a question that comes out of ignorance. When I calculate the half f-stops series using the formula published in the article the resulting series rounded would yield 3.4 instead of 3.3, 5.7 instead of 5.6, 23 instead of 22 etc. I tried to find on google why the rounding seems to follow a rather arbitrary sometimes up sometimes down rule, but I was unable to shed light on this. My obsessive nature would like to know why. — Preceding unsigned comment added by 50.138.183.186 ( talk) 17:33, 9 April 2014 (UTC)
Sorry to come back to such an old topic.
The table below
Standard full-stop f-number scale is not realistic. AFAIK, the fastest lens was the
Carl Zeiss Planar 50mm f/0.7. Hence, we should drop ƒ 0.5. Although I have seen lenses with ƒ 90, they are practically useless due to extreme diffraction (images are very soft). Don't know whether a lens with ƒ 128 exists at all. ƒ 256 is already a
pinhole. Given. But: What puzzles me (like IP 50.138.183.186) is the rounding issue. I understand that there is limited space on a lens to print numbers with more than two significant digits. But why 5.6 instead of 5.7, 22 instead of 23, and 90 instead of 91? I failed to find any reference. Any ideas?
Alfie
↑↓
© 00:23, 1 December 2022 (UTC)
In the "Effects on image sharpness" section, the article makes two contradictory points:
These two points are contradictory because
Assuming that we are using the large-format and the small-format cameras to take the same picture (i.e., same angle of view and subject distance), according to the first point, the entrance pupil diameter (i.e., effective aperture) required to maintain a particular depth of field would remain unchanged. Sure, the small-format camera lens would have a smaller f-number (to match the correspondingly smaller focal length needed to maintain the angle of view), but the aperture size (entrance pupil diameter) would be the same. To clarify, the small-format camera would NOT require a larger aperture than the large-format camera.
I suppose that the second point can also be interpreted in an alternate way: instead of holding the angle-of-view constant (to get the same photo), we can instead hold the focal length constant between the the large- and small-format cameras. However, this comparison between cameras would be unusual since the two cameras wouldn't be taking the same picture: the photo on the small-format camera would appear more zoomed in. Still, even if this alternate interpretation is intended, point 2 does not appear to be true: when the focal lengths and aperture sizes are identical between the large- and small-format cameras, we are more-or-less using the same lens for both cameras, so for the same subject distance, the circles-of-confusion would be identical in size.
If anything, assuming that both the large- and small-format cameras have the same number of pixels, the pixels on the small-format camera would be more closely spaced; as a result, the circle-of-confusion would cover more pixels in the small-format camera, making the blur more obvious there.
Can someone help me double-check this? In any case, the second point (that small-format cameras would require larger apertures to maintain the same depth-of-field) is not obvious; if it stays unchanged, I think it should be supported by a citation.
Best regards, -Jimmy C. Chau 128.197.53.42 ( talk) 22:56, 16 July 2015 (UTC)
I have removed this incorrect parenthetical clause from the article. Jcchau ( talk) 02:58, 24 July 2015 (UTC)
N=f/D is unreliable as a definition of f-number and most often only an approximation. For instance N is not equal to f/D in the typically shown thin lens diagram, as explained by Kingslake: "It is a common error to suppose that the ratio D/2f is actually equal to tan(theta), and not sin(theta)" [1]. So assuming f/stop = f/D maybe fine as a rule of thumb in the field, but fails when put to the test. For instance it may induce people to assume that N can be made arbitrarily small simply by making lens diameter arbitrarily large. Or that one could get an arbitrarily low equivalent f/stop by mounting a reference lens on an arbitrarily small sensor. Both of which are physically impossible for a practical photographic lens.
N is on the other hand always equal to 1/[2sin(theta)] in air for a practical photographic lens that produces reasonable images, per Kingslake and others [2]. Knowing the precise definition of N allows a photographer to decode advanced optics formulas and to answer questions like "what's the smallest f-number possible for a reasonable photographic lens?" (Obvious answer: minimum N = 0.5 in air). That's not possible with the article as it stands.
Therefore I would suggest specifying in the initial definition that N = f/D is a useful approximation that works fine most of the time in the field. But I would also add a section with the precise definition of f-number N = 1/[2nsin(theta')], with n the index of refraction of the medium between the lens and the image plane. That's air in interchangeable lens systems, so n=1. It should then be apparent that N = 1/2NA exactly, with NA = Numerical Aperture.
Can page admins make the relative changes? Jack Hogan ( talk) 16:54, 3 March 2016 (UTC)
Numerical_aperture#Numerical_aperture_versus_f-number explains tan(θ) vs. sin(θ). Gah4 ( talk) 16:18, 13 September 2018 (UTC)
References
On a camera, the aperture setting is usually adjusted in discrete steps, known as f-stops. This was usually true from the rangefinder days through the manual focus SLR days. Though there are detents at full stops, the aperture changes continuously. The later electronic SLRs, and continuing into digital SLRs, normally allow steps of 1/2 or 1/3 stop. The settings are electronic, and not continuous. Seems like we should at least take out the usually. Gah4 ( talk) 07:55, 20 July 2016 (UTC)
Thanks, page useful. Will read in detail, but at this point just trying to help daughter into using the loaned camera. THe split image showing effect of f-number - great. Below a just as useful image with caption. "Shallow focus with a wide open lens" - if this just said (high f-number) or (low f-number) it would be great, much easier to grasp by those reading the page to learn. I totally, understand that if I already knew this all, read the page thoroughly, it would be redundant infromation, but that's why I'm reading it. As a wonderfully informative page, it would be even easier with this consistency of comment in the caption as well. — Preceding unsigned comment added by 122.60.105.241 ( talk) 23:53, 25 August 2018 (UTC)
Can someone rewrite this section? I understand many aspects of photography, but I did not understand this section. Could someone please write it in layperson language, clarify what is being said, and expand it a bit to make it more useful? because it looks like it has the potential of being a very useful section! Misty MH ( talk) 03:43, 10 November 2018 (UTC)
Regarding the correction for objects not at infinity, with TTL (through the lens) metering systems, this will automatically be corrected. Should this be mentioned? Gah4 ( talk) 04:33, 10 November 2018 (UTC)
The article says Most old cameras had a continuously variable aperture scale. I suppose this depends on how old you mean by old. Many box cameras from the early 1900's have a sliding metal tab, or rotating metal disk, with holes in it. The continuous aperture might have been from about the 1930's or so. Many cheaper cameras only offer one aperture setting. With electronic aperture control, the continuous scale went away. Gah4 ( talk) 04:46, 27 March 2019 (UTC)
@ Rkinch: Regarding this edit, please provide a page reference in the book cited, and a quote. I do not believe the new definition you have put in the article is correct.-- Srleffler ( talk) 03:48, 12 August 2019 (UTC)
...energy collected from a small area of the object is directly proportional to the area of the clear aperture, or entrance pupil, of the lens. At the image, the illumination (power per unit area) is inversely proportional to the image area over which this object is spread. Now the aperture area is proportional to the square of the pupil diameter, and the image area is proportional to the square of the image distances, or focal length (f). Thus, the square of the ratio of these two dimensions is a measure of the relative illumination produced in the image.
The ratio of the focal length to the clear aperture of a lens system is called the relative aperture, f-number, or "speed" of the system, and...
A recent change removes photography as the use of f/number. As well as I know, optics other than photography, uses Numerical Aperture, symbolized as N.A. In most cases, N.A. is twice the denominator of the f/number. (That is, if you think of it as a fraction.) Is there a WP:RS that shows it widely used in optics, other than photography, or I suppose when photographic lenses are used in non-photographic applications. Gah4 ( talk) 01:22, 9 August 2020 (UTC)
More specifically, this place:
″A 100 mm focal length f/4 lens has an entrance pupil diameter of 25 mm. A 100 mm focal length f/2 lens has an entrance pupil diameter of 50 mm. Since the area varies as the square of the pupil diameter, the amount of light admitted by the f/2 lens is four times that of the f/4 lens. To obtain the same photographic exposure, the exposure time must be reduced by a factor of four.
A 200 mm focal length f/4 lens has an entrance pupil diameter of 50 mm. The 200 mm lens's entrance pupil has four times the area of the 100 mm lens's entrance pupil, and thus collects four times as much light from each object in the lens's field of view. But compared to the 100 mm lens, the 200 mm lens projects an image of each object twice as high and twice as wide, covering four times the area, and so both lens produce the same illuminance at the focal plane when imaging a scene of a given luminance.″
The excerpt's second paragraph doesn't mention what f-number of the compared 100mm focal length lens is? Is it from the previous example of the first paragraph? If yes, is it f/2 or f/4 100 mm lens? Be more specific and detailed about such details, please.
"The 200 mm lens's entrance pupil has four times the area of the 100 mm lens's entrance pupil, and thus collects four times as much light from each object in the lens's field of view" - why? Expand on that with a couple of words more (by including relevant information in the article, not here).
Also, going back to the same example: "Since the area varies as the square of the pupil diameter" - why? This step occurred abruptly after the preceding one without any transition explaining how the square was come up with. Scrutinizer798 ( talk) 01:13, 23 September 2020 (UTC) Scrutinizer798 ( talk) 01:03, 23 September 2020 (UTC)
The article currently reads, "Modern electronically controlled interchangeable lenses, such as those used for SLR cameras, have f-stops specified internally in 1⁄8-stop increments, so the cameras' 1⁄3-stop settings are approximated by the nearest 1⁄8-stop setting in the lens." I'd really love to see a citation for this! It seems really unlikely, given that aperture is often controlled by a solenoid on cheap cameras, and a stepper motor on more expensive ones. 71.63.160.210 ( talk) 09:01, 27 December 2021 (UTC)
(I know essentially nothing about photography, and I think this article does an astonishingly good job of explaining its complex topic.)
In a couple of places, it is pointed out that certain assumptions are only strictly valid when the subject of the photograph is infinitely far away, and therefore those assumptions must be modified when the subject is close to the lens. But how close is "close"? In other words, at what distance from the lens does it become necessary to compensate for the closeness of the subject? I imagine it might be proportional to certain characteristics of each lens. TooManyFingers ( talk) 16:14, 19 September 2023 (UTC)