View source for Talk:F-number

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== A Spatial Filter ==
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?[[User:Waxsin|Waxsin]] ([[User talk:Waxsin|talk]]) 16:10, 2 February 2016 (UTC)

:It is not intended as a spatial filter, and it is not at the focus of the lens. It does in some way behave as a low-pass filter, at small apertures, when sharpness is limited by diffraction. —&nbsp;[[User:Edgar.bonet|Edgar.bonet]] ([[User talk:Edgar.bonet|talk]]) 18:29, 2 February 2016 (UTC)

== Definition of lens brightness ==
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.  [[User:Balazer|Balazer]] ([[User talk:Balazer|talk]]) 21:05, 15 January 2013 (UTC)

:As far as standard units are concerned, we should strive for a more consolidated presentation of the various units and their conversions. 
:Regarding lens brightness, as I understand it ... Roughly speaking, compared to a smaller lens aperture, the geometry of a larger lens aperture focusing on the same image frame will allow more light to enter the lens '''''and''''' be projected onto the sensor/film/eye-piece. Think of aperture size like the size of a bucket for bailing out a leaking boat. Say you pour water out of a leaking boat at the same rate (pours per second) no matter what sized bucket you have. The larger the bucket, the more water you get each time. 
:The more light that can enter through the lens '''''and''''' arrive at the sensor/film/eye-piece, as filtered through the lens, the brighter the image and that's what is meant, in generic terms, by lens brightness. Lens material, component geometries, and composite efficiencies all effect light loss and thereby, lens brightness (on this final note, I have to acknowledge that I may not be using the standard terminology). Don't yell at me internet. [[User:JimsMaher|JimsMaher]] ([[User talk:JimsMaher|talk]]) 17:34, 17 January 2013 (UTC)
::Better yet ... Think of aperture size like a sieve straining rocks and debris. The larger the holes, the looser the sieve, the more stuff will fall through. The smaller the holes, the denser the sieve, the more solid material making up the sieve, the less debris is allowed through. A sieve that stops a lot of material from going through corresponds to a large f number (f/45, f/64). A sieve that's no more than a metal ring, for sake of example, corresponds to a lens with a low f number (f/1.4, f/1). [[User:JimsMaher|JimsMaher]] ([[User talk:JimsMaher|talk]]) 17:49, 17 January 2013 (UTC)

:Balazer has it right.  I haven't looked to see if  have good refs to back that up, but they exist (e.g. [http://books.google.com/books?id=EYi1iGRBM6kC&pg=PA65#v=onepage&q&f=false this page]).  The resulting ratio (in steradians) is inversely proportional to the square of the f-number.   [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 18:22, 17 January 2013 (UTC)
::Since the light is passing through a circle (for sake of example), with light intensity following the [[Inverse-square law]], every halving of the diameter of the aperture corresponds to a doubling of the distance from a fixed light source, in terms of exposure value. Or atleast that's the parsimonious model. [[User:JimsMaher|JimsMaher]] ([[User talk:JimsMaher|talk]]) 20:02, 17 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. [[User:BigslyE5|BigslyE5]] ([[User talk:BigslyE5|talk]]) 15:09, 26 July 2014 (UTC)

And that is why <math>F/N=D</math> or focal length divided by f-stop equals Diameter.[[User:19dreiundachtzig|19dreiundachtzig]] ([[User talk:19dreiundachtzig|talk]]) 00:35, 18 December 2020 (UTC)

==Aperture vs. Entrance Pupil in the definition of f-number==
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''. [[User:Balazer|Balazer]] ([[User talk:Balazer|talk]]) 04:30, 19 January 2013 (UTC)

:I suppose, but for the early cameras, when f-number was being defined, with fairly simple lenses and the aperture very close to the lens, it would have been close enough. With modern [[Angénieux_retrofocus|retrofocus]] lenses, it is much less obvious, so entrance pupil is better. But unless someone actually takes the lens apart, they won't know the actual size of the aperture! [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 20:38, 7 February 2013 (UTC)

::But there are common real-world examples where the the distinction really matters and the difference is easy to see.  Take most any video zoom lens, or any other zoom lens with an f-number that is constant across the zoom range.  The physical aperture is unchanged as you zoom, but the entrance pupil gets larger as you zoom in.  It's quite obvious if you look into the front of the lens as you increase the focal length:  the image of the aperture becomes increasingly magnified.  [[User:Balazer|Balazer]] ([[User talk:Balazer|talk]]) 04:59, 10 March 2013 (UTC)

:::Seems to me, though, that aperture is the [[WP:COMMONNAME]] used by photographers, even though it is technically wrong.  As you note the physical aperture can change, such as in zoom lenses, but photographers never say "physical aperture", just "aperture". It seems that "effective aperture" is used for antennas, and from [https://www.merriam-webster.com/dictionary/effective%20aperture Webster], also used in place of entrance pupil in photography. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 16:04, 13 September 2018 (UTC)

: I would suggest using the term 'objective lens,' or just 'objective' rather than the term 'entrance pupil,' as pupils generally have a (biologic and common) definition that includes adjusting in size to adapt the eye to the amount of light entering. 'Objective lens,' or '[[Objective]]' are both used for microscopes and telescopes, which relates nicely to camera lenses. [[User:DocKrin|DocKrin]] ([[User talk:DocKrin|talk]]) 18:07, 11 August 2019 (UTC)
::Sorry, but you don't seem to understand what is being discussed here. The [[entrance pupil]] of an optical system is the image of the system's [[aperture stop]], formed by whatever optics are in front of the stop. The system's f-number is the ratio of its focal length to the diameter of the entrance pupil. When you look into someone's eye, the image of their [[pupil]] that you see is in fact the entrance pupil of the eye; the terminology is not a coincidence.--[[User:Srleffler|Srleffler]] ([[User talk:Srleffler|talk]]) 22:43, 11 August 2019 (UTC)

== Higher or Lower? ==

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.  [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 09:12, 4 March 2013 (UTC)

:I'm not quite sure if you are asking a question, but f/8 is definitely a larger aperture than f/11; going from f/11 to f/8 would be increasing the aperture. One could say it is also "reducing the f/#", but that's not common and is asking for ambiguity. [[User:BenFrantzDale|—Ben FrantzDale]] ([[User talk:BenFrantzDale|talk]]) 21:31, 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. [[User:JimsMaher|JimsMaher]] ([[User talk:JimsMaher|talk]]) 14:35, 7 March 2013 (UTC)
:No, it is not ambiguous, and not uncommon to say "increase the aperture" (meaning to open up); see [https://www.google.com/search?q=%22increase+the+aperture%22&aq=f&oq=%22increase+the+aperture%22&aqs=chrome.0.57j0j62.4517&sourceid=chrome&ie=UTF-8#q=%22increase+the+aperture%22&hl=en&source=lnms&tbm=bks&sa=X&ei=arY4UfSNJYLDyAGQ-oDQBw&ved=0CA8Q_AUoAg&bav=on.2,or.r_gc.r_pw.r_cp.r_qf.&bvm=bv.43287494,d.aWc&fp=915d6507b0a4bb1d&biw=1271&bih=668 books].  The aperture is the hole; increasing it can only mean making it bigger.  [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 15:49, 7 March 2013 (UTC)
::OK, books. But that's not my experience. In verbal communication, there's a different set of vocabulary used. When efficiency of communication is a priority, then any terminology that is known to be easily misunderstood is actively avoided in diverse climates.  It is ambiguous for photographic purposes, varying the aperture serves multiple purposes ([[Exposure value|EV]], [[Depth of Field|DOF]], compensating for [[vignetting]] or [[Chromatic aberration|CA]], etc.) not all of which lead to the same understanding. And I didn't say it was uncommon, I just suggested using a more universally understandable phrasing. But to be clear, that phrase is foreign to my ear. [[User:JimsMaher|JimsMaher]] ([[User talk:JimsMaher|talk]]) 19:42, 7 March 2013 (UTC)
:::It's only confusing if you conflate ''aperture'' and ''f-number''.  An aperture is an opening.  An f-number is a ratio.  Increasing the aperture must be taken to mean increasing the size of the aperture.  Yes, it has the opposite effect of increasing the f-number.  Saying ''increase the aperture'' is common in my experience, and is always understood to mean choosing a lens or aperture setting with a smaller f-number. [[User:Balazer|Balazer]] ([[User talk:Balazer|talk]]) 05:16, 10 March 2013 (UTC)
::::F-number confusion is another matter (referring to the misunderstanding of the fractional representation of f-numbers).  I'm referring to the general 'conflation' of two properties, whereby the increasing of one property is positively related to some other property. Compared to the negative correlation of properties. Negative relations are less intuitive, that's all I was addressing. Unless you're suggesting that the phrase "increase the aperture" is the preferred way to tell someone to increase the f-number, i.e. widen the aperture ... but I don't think anyone here is saying that. 
::::I suggest there is potential confusion with either phrase, "increase the F-stop" or "increase the aperture". Which is unfortunate. [[User:JimsMaher|JimsMaher]] ([[User talk:JimsMaher|talk]]) 19:02, 11 March 2013 (UTC)
:::::Indeed, it's better to be explicit and say "increase the F-number" or "increase the aperture diameter".  [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 21:07, 11 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. [[User:BigslyE5|BigslyE5]] ([[User talk:BigslyE5|talk]]) 14:55, 26 July 2014 (UTC)
:Since I started this one, I will add one more comment. It seems to me that photography does it one way and optics the other way. I am not so sure that there is a good reason for that, but it occurred to me while reading an optics book. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 18:57, 17 July 2015 (UTC)

::So, I have to say, no, it's not written technically correct.  The f number is the whole "f/2", not just the "2".  f/2 is larger than f/8 (plug in 50 for the focal length's f to each: for f/2, you'll get 50/2 which 25 fully reduced, and for f/8 you'll get 50/8 which is 12.5.  25>12.5). It's important to maintain this connection because 25mm is the actual size of the aperture when a 50mm lens is set to f/2.  By calling f/8 "bigger" than f/2, you're divorcing the concepts from each other, while also introducing a weird inversion that is a source of confusion for many people which muddles communication and carries on to understanding the relationship between focal length, aperture (as in the apparent size of the opening), and the amount of light that passes through.  This is an important enough concept that not only should the article be corrected, but there should also be a section included directly addressing this common mistake.[[Special:Contributions/107.77.235.71|107.77.235.71]] ([[User talk:107.77.235.71|talk]]) 22:15, 8 April 2019 (UTC)

:::That is the way I thought of it, and why I started this section. Optics uses N.A. (numerical aperture) instead of f/number, but most often photographers says ''aperture'', and not ''eff-number''.  Most of the article considers the number below the f, and not the fraction. A similar problem comes up with shutter speeds, where often enough, one indicates the number in the denominator.  (In both cases, that is what is printed on the lens and camera.)   Continuing, faster shutter speeds have larger denominators, or smaller values. A speed should be a value with time in the denominator!  Anyway, as written the article seems to use the number below the f, not the fraction.  I don't know if there is enough of a consensus to change it. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 00:38, 9 April 2019 (UTC)

::::What change are you thinking of, and what sources back it up? [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 02:46, 9 April 2019 (UTC)

::::I just took another pass through books, and they universally say the f-number is f/D, and that small f-numbers correspond to large apertures. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 02:51, 9 April 2019 (UTC)

:::::Which books are these?  Not that I disagree, as I mostly don't read books where the question would come up.  Photographers that I know say ''set the aperture to eff two''.  Maybe followed by ''decrease the aperture to eff four''.  I sounds strange to say ''set the eff number to 11'', and I pretty much don't know anyone to use the phrase ''eff number''.  Maybe ''what eff-stop did you use'', but not ''what did you set the eff number to''?  That is more for what photographers say, and less what they might write in books. Phrases like ''full aperture'', ''minimum aperture'' and ''maximum aperture'' are common.  (I read books on darkroom work more than camera work, but enlargers also have an aperture ring.)  [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 04:13, 9 April 2019 (UTC)
::::::Just scanning through [https://www.google.com/search?tbm=bks&q=%22f-number%22+%22aperture%22 these book hits] and  [https://www.google.com/search?biw=1512&bih=894&tbm=bks&q=lower+f-number+aperture and these].  [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 18:50, 9 April 2019 (UTC)
::::::The term "f-stop" is actually [https://books.google.com/ngrams/graph?content=f-stop%2Cf-number%2Cf+stop%2Cf+number&year_start=1900&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cf%20-%20stop%3B%2Cc0%3B.t1%3B%2Cf%20-%20number%3B%2Cc0%3B.t1%3B%2Cf%20stop%3B%2Cc0%3B.t1%3B%2Cf%20number%3B%2Cc0 more commonly used], but has a hard time finding an agreed definition.  [https://books.google.com/books?id=ZkOYrPiIwFIC&pg=PA52&dq=f-stop+ratio+aperture&hl=en&sa=X&ved=0ahUKEwjEqr_K2cPhAhWkJjQIHRhlAW4Q6AEIYDAJ#v=onepage&q=f-stop%20ratio%20aperture&f=false Here], for example, the f-stop is the reciprocal of the f-number.  More commonly the [https://www.google.com/search?tbm=bks&hl=en&q=%22f-stop+value%22 "f-stop value"] or [https://www.google.com/search?tbm=bks&hl=en&q=%22f-stop+measurement%22 "f-stop measurement"] or [https://www.google.com/search?tbm=bks&hl=en&q=%22f-stop+setting%22 "f-stop setting"] or [https://www.google.com/search?tbm=bks&hl=en&q=%22f-stop+number%22 "f-stop number"] is defined like what's commonly called "f-number".  Of these, [https://books.google.com/ngrams/graph?content=f-stop+*%2Cf-number&year_start=1900&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t2%3B%2Cf%20-%20stop%20%2A%3B%2Cc0%3B%2Cs0%3B%3Bf%20-%20stop%20and%3B%2Cc0%3B%3Bf%20-%20stop%20number%3B%2Cc0%3B%3Bf%20-%20stop%20of%3B%2Cc0%3B%3Bf%20-%20stop%20is%3B%2Cc0%3B%3Bf%20-%20stop%20to%3B%2Cc0%3B%3Bf%20-%20stop%20numbers%3B%2Cc0%3B%3Bf%20-%20stop%20setting%3B%2Cc0%3B%3Bf%20-%20stop%20for%3B%2Cc0%3B%3Bf%20-%20stop%20or%3B%2Cc0%3B%3Bf%20-%20stop%20on%3B%2Cc0%3B.t1%3B%2Cf%20-%20number%3B%2Cc0 "f-stop number" is most common], but not nearly as common as "f-number".  [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 19:18, 9 April 2019 (UTC)

:::::::The one you call [https://books.google.com/books?id=ZkOYrPiIwFIC&pg=PA52&dq=f-stop+ratio+aperture&hl=en&sa=X&ved=0ahUKEwjEqr_K2cPhAhWkJjQIHRhlAW4Q6AEIYDAJ#v=onepage&q=f-stop%20ratio%20aperture&f=false Here] seems to agree with me. It mentions ''aperture'' 96 times, ''f-stop'' not quite as many, but (unless I missed it) ''f-number'' only once. Seems to me from that, the the [[WP:COMMONNAME]] should be aperture or f-stop. I didn't look at the other ones yet. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 20:42, 9 April 2019 (UTC)
::::::::My point about that source is that it's an outlier, with a definition of f-stop that's rare, or possibly unique.  And the article is about f-number; f-stop and aperture are not alternative names for f-number. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 20:59, 9 April 2019 (UTC)

:::::::::[[f-stop]] redirects here.  [[aperture]] has its own article, which duplicates much of this article. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 21:33, 9 April 2019 (UTC)

:::::::::Also, the article starts with ''The f-number of an optical system (such as a camera lens'', though every where else in optics, except for camera lenses, [[numerical aperture]] is used. That page does explain that camera lenses are described differently. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 21:36, 9 April 2019 (UTC)

::::::::::Well, yes, since f-stops are quantified by f-numbers, that makes sense to have that redirect.  Where are you seeing the bit about "every where else in optics"?  [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 21:58, 9 April 2019 (UTC)

::::::::::Oops, I misinterpreted; that's your impression, not a quote.  An opposite impression might come from [https://books.google.com/books?id=kxICp6t-CDAC&pg=PA556#v=onepage&q&f=false this optics book], in which f-number is primary and not camera-specific. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 22:36, 9 April 2019 (UTC)

:::::::::::Sometime after I started this section, I found my actual optics book, which is an earlier edition of [https://www.amazon.com/Optics-4th-Eugene-Hecht/dp/0805385665 this book].  But also, [[numerical aperture]] and has a section [[Numerical_aperture#Numerical_aperture_versus_f-number]] explaining how photography is different. I have worked in optics labs, though don't remember at the time thinking about it one way or the other. I suppose some optics books would be written by photographers, and so describe some things that way. By ''optics'', I mean optical bench laboratory research, and the books that go along with them. There are a lot more photographers than optics researchers, though. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 22:56, 9 April 2019 (UTC)

::::::::::::G, I sense that we have a lot of common interests.  Shoot me an email if you'd like to discuss offline, or if you'd like to come visit and look through my collection of photography and optics books and such (if you're in the Silicon Valley area at some point).  For starters, here's a little historical survey I did:  [http://www.dicklyon.com/tech/Photography/DepthOfField-Lyon.pdf "Depth of Field Outside the Box"], which will give you an idea on some of the relevant sources. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 16:54, 10 April 2019 (UTC)

:::::::::::::Thanks for the link to the paper. Reminds me that when I was young (10), I had a Kodak Autographic Junior 1A from my grandfather, which uses 116 film.  I did get a roll for it (easier to find then) and after not so long, realized that it had US (I didn't know the name at the time) aperture numbering. My father and I then tried to guess how they correspond to f/stops, but I think we were wrong. (But that was VP, with good exposure latitude.)  Not long after that, I inherited much more of my grandfather's photographic equipment, including light meters.  I still have the 1A, and have used it not so long ago.  I wonder now if US didn't take hold, as the numbers get too big (or too small). Note also that some older Kodak cameras just number them 1, 2, 3, 4.  (That is, no system at all.) [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 00:42, 13 April 2019 (UTC)
::::::::::::::OK, we're more aligned than I realized.  The one at [[:File:No1-A Autographic Kodak Jr.jpg]] is the one my granddad bought in 1922 to take pictures of my dad when he was born.  Size 116 film was still easy to find in the 1960s, so that's the camera I learned on.  But the US stops were a puzzle.  I finally found, in the local public library, an old Boy Scouts Photography Merit Badge booklet that had the conversion.  So I made a table on the typewriter and taped it on the camera.  When I was messing with it in my sister's Kodak collection a few years ago, I took the picture.  I recently bought a pack of long-expired size 116 Kodacolor if you'd like to give it a try.  I think I still have a developing tank to fit.  Anyway, I think US stops failed because their definition was too arbitrary; a simple ratio just made more sense and was easy to remember. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 03:30, 13 April 2019 (UTC)

== Early mentioning of EV ==
 
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 [[photography|photographic]] property. 

[[User:Glockenklang1|Glockenklang1]] ([[User talk:Glockenklang1|talk]]) 10:06, 14 July 2013 (UTC)

:I believe the details of the relationship between f-number, exposure time, and exposure value are better left in the Exposure Value article, and not in the f-number article.  Exposure value is a concept quite specific to photography, and not about lenses in general.  Exposure value doesn't apply to binoculars, microscopes, telescopes, projectors, and other optical systems where there is no shutter or camera.  In the f-number article we already explain the concept of lens brightness, and relate it to image brightness and exposure.  I believe that's sufficient.  [[User:Balazer|Balazer]] ([[User talk:Balazer|talk]]) 00:41, 15 July 2013 (UTC)

::May be I am just stumbling about the wording 'important concept', which is rather a 'starter' but is lacking the impact within the intro - However, there is already enough content lateron.  [[User:Glockenklang1|Glockenklang1]] ([[User talk:Glockenklang1|talk]]) 18:30, 15 July 2013 (UTC)
:::Should we just remove the "important concept" bit?  I always thought it was awkward wording.  I usually think the part before the article contents should be a straightforward definition without any editorializing, though I'm not sure what Wikipedia style guides say about the matter. [[User:Balazer|Balazer]] ([[User talk:Balazer|talk]]) 18:44, 15 July 2013 (UTC)
::The introduction is quite short for the length of the article, I would like to see one or two sentences about why we have this definition [for the beginning photographer]. The article is hard to digest. [[User:Glockenklang1|Glockenklang1]] ([[User talk:Glockenklang1|talk]]) 19:49, 15 July 2013 (UTC)

:::As I know it, EV is a combination of aperture and shutter speed. It allows for one number to indicate any of the combinations that allow for equivalent exposure. However, changes to either aperture or shutter speed give the equivalent change in EV.

==Two references (google books) after searching for ''effective aperture'' and ''relative aperture'' (f/ number) ==

[http://books.google.se/books?id=SyW3Pb9qqrkC&pg=PA38&lpg=PA38&dq=what+is+effective+aperture+relative+aperture+lens&source=bl&ots=XYuv_2eQWF&sig=WWLIvcf-UOPGyQrNNUImrIkM4xA&hl=en&sa=X&ei=f0nSUorhF4Se4gTt3oHAAg&redir_esc=y#v=onepage&q=what%20is%20effective%20aperture%20relative%20aperture%20lens&f=false  Nanette L. Salvaggio  ''Basic Photographic Materials and Processes'']

[http://books.google.se/books?id=cuzYl4hx-B8C&pg=PA127&lpg=PA127&dq=effective+aperture+lens+ray&source=bl&ots=n0OxzTBCLC&sig=UWbd9uefVmHm5xQ4knxs-nVyVF0&hl=en&sa=X&ei=7U_SUr6BLIj44QSIgYHYAg&redir_esc=y#v=onepage&q=effective%20aperture%20lens%20ray&f=false  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. 

[[User:RPSM|RPSM]] ([[User talk:RPSM|talk]]) 08:44, 12 January 2014 (UTC)

== F-stops rounding error? ==

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.  <span style="font-size: smaller;" class="autosigned">— Preceding [[Wikipedia:Signatures|unsigned]] comment added by [[Special:Contributions/50.138.183.186|50.138.183.186]] ([[User talk:50.138.183.186|talk]]) 17:33, 9 April 2014 (UTC)</span><!-- Template:Unsigned IP --> <!--Autosigned by SineBot-->

:I now see in the article that... "For all practical purposes extreme accuracy is not required (mechanical shutter speeds were notoriously inaccurate as wear and lubrication varied, with no effect on exposure)." - OK, that seems to explain it more or less.

::Seems to me that they are powers of two, and powers of two times 1.4 and rounded. More decimal digits is more than has any use, for the reasons noted above. This was all done in the time of slide rules, or, with some luck, mechanical calculators. Once people get used to the values, why change them? I have known lenses with 32 and 45 on them. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 09:45, 4 May 2014 (UTC)
{{outdent}}
[[File:Zeiss_Planar_50mm_F0.7_lens_attached_to_camera..jpg|thumb|right|Zeiss Planar ƒ 1:0.7 50&nbsp;mm used in [[Stanley Kubrick|Kubrick's]] [[Barry Lyndon]]]]
Sorry to come back to such an old topic.<br />The table below '''[[F-number#Standard_full-stop_f-number_scale|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 camera|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?<br />[[User:Alfie66|<span style="font:600 small-caps Verdana; color:#fff;background-color:#955; padding:0 3px;">Alfie</span>]][[User Talk:Alfie66|<span style="color:#fff;background-color:#559; padding:0 3px;">↑↓</span>]][[Special:Contributions/Alfie66|©]] 00:23, 1 December 2022 (UTC)
:
:First, it is not unusual that the largest aperture (smallest number) is not on of the standard values, and is marked on the lens.
:Otherwise, I suspect that the tolerance on building lenses means that two digits is plenty. If you use 1.4 as an approximate (two digit) value for sqrt(2), you get the usual values, including 45. Once people started using some set of numbers, there would be tendency to keep using them. I believe the lens with 45 is designed for use with an extension tube, and two stop loss, and is numbered accordingly. 
:Note also that the values only represent the amount of light when focused to infinity. For ordinary distances, it will still be close enough to one [[sigfigs|sigfig]]. At extreme close-up distances, even that isn't close. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 09:14, 1 December 2022 (UTC)
::I certainly agree with most of your points. Can you please outline how one would arrive at 5.6, 22, 45, and 90? I don’t want to dive into [[WP:OR]]. Rather I'm interested how/why/when these numbers entered the scene. [[User:Alfie66|<span style="font:600 small-caps Verdana; color:#fff;background-color:#955; padding:0 3px;">Alfie</span>]][[User Talk:Alfie66|<span style="color:#fff;background-color:#559; padding:0 3px;">↑↓</span>]][[Special:Contributions/Alfie66|©]] 15:46, 2 December 2022 (UTC)
:::
:::Fortunately, [[WP:OR]] is allowed in talk pages. 4*1.4, 8*1.4, 16*1.4, 32*1.4, rounded to two digits. If you use 1.41, they don't round to those values. Note that [[WP:CALC]] allows for calculations. We don't know when these values appeared, or who did it. There were other numbering systems around in the early years, including the U.S. (not United States) system. I have some cameras that use it. In U.S., values increase as the area decreases. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 01:28, 3 December 2022 (UTC)

== Effects on image sharpness ==
In the "Effects on image sharpness" section, the article makes two contradictory points:
# Depth of field can be described as depending on just angle of view, subject distance, and entrance pupil diameter (as in von Rohr's method). 
# Therefore, reduced–depth-of-field effects will require smaller f-numbers (and thus larger apertures and so potentially more complex optics) when using small-format cameras than when using larger-format cameras.

These two points are contradictory because
* point 1 claims that as long as the three variables (angle of view, subject distance, and entrance pupil diameter) remain the same, the depth of field remains the same,
* but the parenthetical clause of point 2 claims that larger apertures are needed for small-format cameras to maintain the depth of field.  

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
[[Special:Contributions/128.197.53.42|128.197.53.42]] ([[User talk:128.197.53.42|talk]]) 22:56, 16 July 2015 (UTC)

:It might be the uncertainty in the meaning of depth-of-field. When I first learned about DOF, it was explained in terms of the abilities of the lens. That is, a better (less aberration) lens will have a smaller circle of confusion, and so smaller depth of field. Eventually, you are diffraction limited. Given all the uncertainties, I believe you can make arguments either way. It is likely not possible to make two lenses of equal 'quality' for the same angle of coverage and different image size. There are too many variables. This is especially interesting considering the lenses available for APS-C format DSLRs, and full frame 35mm lenses, used on the same camera. As noted, there is the additional complication of increased magnification viewing the smaller format image. As well as I know it, DOF is independent of that, depending only on the circle of confusion at the image plane. But others might disagree. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 19:07, 17 July 2015 (UTC)

::I agree with you (Gah4) on some points, but I have to disagree with your explanation of depth of field.  Before proceeding, to avoid misunderstanding, I must clarify the terminology that I use.  I will use
::* "[[circle of confusion]]" to mean "an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source" (copied from the Wikipedia article), and
::* "maximum permissible circle of confusion" to refer to the largest acceptable circle of confusion.  As the [[Circle of confusion]] article states, this is commonly set to the "largest blur spot that is indistinguishable from a point".  Thus, the maximum permissible circle of confusion may depend on the pixel pitch (i.e., the distance between the center of the pixels).  
::Although "maximum permissible circle of confusion" is often shortened to just "circle of confusion", the distinction between the two is important in this case to avoid accidentally swapping the two values.  

::All else being equal, as you state, reducing aberrations will yield a smaller "circle of confusion", but unless this somehow affects the requirements for the imaging system, should not affect the "maximum permissible circle of confusion".  

::You seem to argue that this smaller "circle of confusion" would lead to a smaller depth of field.  However, a smaller depth of field means that less of the scene is in focus (corresponding to a /larger/ circle of confusion (i.e., more blurring) for those parts that are too far or too close to be in focus).  Perhaps you mean that a smaller "maximum permissible circle of confusion" would yield a smaller depth of field.  

::Although aberrations certainly affect how well a lens can focus on a scene, from my understanding, depth-of-field calculations often ignore aberrations for simplicity.  This seems to be the case with the von Rohr method mentioned in the [[f-number]] article, which (as stated in the article) only accounts for angle of view, subject distance, and entrance pupil diameter; these parameters alone are insufficient to specify the lens's aberrations.  Even if we ignore aberrations though, the depth-of-field would still be limited if the aperture was larger than a point.  [[User:Jcchau|Jcchau]] ([[User talk:Jcchau|talk]]) 06:54, 20 July 2015 (UTC)

:::In the case of interchangeable lenses, the lens doesn't know the pixel pitch of the camera it might be attached to, but I learned about depth of field in the film camera days. Film also has a resolution limit, but it is more gradual than digital. If you allow for a Gaussian due to lens effects, and an additional Gaussian due to focus error, the widths add as RMS, so I would set the permissible circle of confusion to sqrt(2) times the lens (diffraction and aberration) circle of confusion. In the case of non-interchangeable lens, you can include the sensor resolution. But non-interchangeable lenses often don't have DOF scales.  [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 17:45, 24 July 2015 (UTC)

I have removed this incorrect parenthetical clause from the article.  [[User:Jcchau|Jcchau]] ([[User talk:Jcchau|talk]]) 02:58, 24 July 2015 (UTC)

== Specify that N = f/D is often a practical approximation.  Include the precise definition of N ==

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 [https://upload.wikimedia.org/wikipedia/commons/thumb/0/08/Numerical_aperture_for_a_lens.svg/200px-Numerical_aperture_for_a_lens.svg.png 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)"<ref> Rudolf Kingslake (1951). Lenses in photography: the practical guide to optics for photographers. Case-Hoyt, for Garden City Books. pp. 97–98.</ref>.  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<ref>[https://books.google.it/books?id=UY6QzgzgieYC&lpg=PP1&dq=nakamura%20The%20F-number%20of%20a%20lens%20(%20F%20)%20is%20expressed%20as%20half%20the%20opening%20angle%20q%C2%A2%20for%20the%20focused%20light%20beams%20in%20Figure%202.1%2C%20as%20shown%20as%20in%20Equation%202.4.&pg=PA25#v=onepage&q=The%20F-number%20of%20a%20lens&f=false Image Sensors and Signal Processing for Digital Still Cameras]</ref>.  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? [[User:Jack Hogan|Jack Hogan]] ([[User talk:Jack Hogan|talk]]) 16:54, 3 March 2016 (UTC)

:Well, we should check the appropriate references, but I have always seen the f-number ''defined'' as {{math|1=''N'' = ''f''/''D''}}. There is no contradiction with {{math|1=''N'' = 1/(2NA)}} because, for any practical lens, {{math|1=''D''/''f'' = 2 sin(θ)}}.
:The above formula obviously doesn't hold for the thin-lens diagram, but a thin lens is ''not'' a practical photographic lens, it's just a very simplified representation which is consistent with the small-angle approximation. So, in the small-angle limit, {{math|sin(θ) ≈ tan(θ)}} and the thin lens diagram can be trusted. For fast lenses, {{math|1=''N'' = ''f''/''D'' = 1/(2 sin(θ))}} still holds as per the [[Abbe sine condition]], but the thin-lens (which implies {{math|1=''D''/''f'' = 2 tan(θ)}}) is wrong.
:—&nbsp;[[User:Edgar.bonet|Edgar.bonet]] ([[User talk:Edgar.bonet|talk]]) 09:44, 4 March 2016 (UTC)
:'''Addendum''': Here is a [http://photo.stackexchange.com/a/56945 derivation of {{math|1=''f''/''D'' = 1/(2 sin(θ))}}] based on the [[Etendue#Conservation of etendue|conservation of etendue]]. And here is a serious comic discussing [http://what-if.xkcd.com/145/ why you can't make a lens arbitrarily fast]. —&nbsp;[[User:Edgar.bonet|Edgar.bonet]] ([[User talk:Edgar.bonet|talk]]) 09:55, 4 March 2016 (UTC)

::Seems to me that f/numbers were defined when lenses were small enough, and focal lengths long enough, that the difference between {{math|sin(θ)}}  and {{math|tan(θ)}} wasn't significant. Now that you mention it, I am not sure how it is actually done in the lenses where it is significant.  Well, the amount of light collected should be proportional to the area of the lens. That is, the solid angle as viewed by the subject. Even more, note that the ratio does not correctly follow the light delivered to the image when focused closer than infinity. Except for extreme close-ups, it is usually close enough, and those doing close-ups know to correct it. Even more, with through-the-lens metering, it mostly doesn't matter. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 07:36, 20 July 2016 (UTC)

[[Numerical_aperture#Numerical_aperture_versus_f-number]] explains tan(θ) vs. sin(θ). [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 16:18, 13 September 2018 (UTC)

{{reflist_talk}}

== step sizes ==

''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''.   [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 07:55, 20 July 2016 (UTC)

== Shallow focus with a wide open lens - Just a comment ==

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.  <!-- Template:Unsigned IP --><small class="autosigned">—&nbsp;Preceding [[Wikipedia:Signatures|unsigned]] comment added by [[Special:Contributions/122.60.105.241|122.60.105.241]] ([[User talk:122.60.105.241#top|talk]]) 23:53, 25 August 2018 (UTC)</small> <!--Autosigned by SineBot-->

:As the comments above indicate, photographers pretty much never say high or low, or increase, or decrease, the f-number. Expressions like wide open, open up, or stop down are common, and everyone very quickly learns that open up is a smaller number written on the lens, and stop down is a larger number.  Even more, though, photographers pretty much always write it in the f/(number) fraction form, such that increasing (number) decreases the fraction. There just isn't enough room for the f/ on the lens barrel. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 16:26, 13 September 2018 (UTC)

== Working f-number rewrite? ==

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! [[User:Misty MH|Misty MH]] ([[User talk:Misty MH|talk]]) 03:43, 10 November 2018 (UTC)

:Doesn't look so bad to me.  f/number is defined as the focal length divided by the (effective) lens diameter.  (Entrance pupil in more complicated lenses.)  That gives the appropriate amount of light when the lens is focused (and the object is at) infinity (or close enough).  But as you focus closer, and the lens moves farther from the image plane, the light gathering ability reduces.   It is, pretty much, based on the new lens to image plane distance.  Most of the time it can be ignored, but with extension tubes or bellows, the lens can be much farther away, and the correction gets more important.  That is why it is commonly the bellows correction factor.  [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 04:31, 10 November 2018 (UTC)
::It doesn't look so bad because I edited it between when he posted and when you replied.--[[User:Srleffler|Srleffler]] ([[User talk:Srleffler|talk]]) 16:08, 10 November 2018 (UTC)

== TTL ==

Regarding the correction for objects not at infinity, with TTL (through the lens) metering systems, this will automatically be corrected. Should this be mentioned? [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 04:33, 10 November 2018 (UTC)

== ''Most old cameras had a continuously variable aperture scale,'' ==

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. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 04:46, 27 March 2019 (UTC)
:Actually, even modern electronically controlled lenses have an essentially continuously variable aperture setting.  I don't recall the details, but I seem to recall the increments are tiny; it's just the camera UI that quantizes them coarsely. But more generally, you're right, there has always been a mix.  But for cameras with iris diaphragms, they were usually continuous. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 04:34, 12 August 2019 (UTC)

== Definition ==

{{ping|Rkinch}} Regarding [https://en.wikipedia.org/w/index.php?title=F-number&curid=168244&diff=910441347&oldid=910412713 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.--[[User:Srleffler|Srleffler]] ([[User talk:Srleffler|talk]]) 03:48, 12 August 2019 (UTC)
:Nevermind. From Google books snippets it's clear that Smith defines "clear aperture" as a synonym for "entrance pupil". Since we have an article on the latter and it is the more common term in optical design, I'm reverting your change.--[[User:Srleffler|Srleffler]] ([[User talk:Srleffler|talk]]) 03:52, 12 August 2019 (UTC)
:From p. 183 in Smith's book (emphasis added): {{blockquote|...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.<p>''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...}}
:It's not wrong to define "clear aperture" as a synonym for "entrance pupil" as Smith has done, but if you want to do that you have to define the term. Linking it as "clear [[aperture]]", as in the edit I reverted, is simply wrong. The entrance pupil diameter ("clear aperture") is related to the physical aperture diameter, but they are not the same thing. The f-number is defined in terms of the former, not the latter.--[[User:Srleffler|Srleffler]] ([[User talk:Srleffler|talk]]) 04:19, 12 August 2019 (UTC)
::"Clear aperture" is the essential concept, the sound and logical foundation, and why Smith uses that term, as should the WP article.  "Entrance pupil" is a more complex concept dealing with multi-element designs and the theory of stops.  "Entrance pupil" is not the concept underlying relative aperture, although of course it does determine the *calculation* in multiple elements.  The two terms are not equivalent (aperture != pupil, else why would we have redundant terms), although the evaluation may be.  We don't call it "relative pupil".  In terms of an encyclopedic approach, the intro should state the concept in its essential terms (aperture vs focal length), and then the detailed section can refine how the pupil may differ from the physical aperture.  An intelligent but non-expert reader is not going to understand "entrance pupil", but "clear aperture" is immediately understandable.  Intro'ing with "entrance pupil" is going to lose most readers at the start.  The article is a mess of confusion because it's trying to speak in terms of photography some places and geometric optics in others, and this pupil/aperture conflict is one case of that confusion.  You and I as editors may understand the difference between a pupil and an aperture, but we cannot assume that expertise in whoever is reading the intro.  The biggest fault in this article is that it correctly defines the f/number as a property of an ideal lens (circular apertures, zero aberrations), and then proceeds to indoctrinate the idea that real lenses have a perfectly precise, one-dimensional constant f-number, which they don't.  This is similar to the confusion of resolution with MTF, likewise a reduction of the truth. [[User:Rkinch|Richard J Kinch]] ([[User talk:Rkinch|talk]]) 22:14, 13 August 2019 (UTC)
:::Something like "clear aperture ([[entrance pupil]]) diameter" might be a good compromise? [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 18:59, 12 August 2019 (UTC)
::::Yes, that would reserve the distinction and avoid the reduction, which could be explained later in detail.  Perhaps qualify [[entrance pupil]] with the word "typically". [[User:Rkinch|Richard J Kinch]] ([[User talk:Rkinch|talk]]) 22:14, 13 August 2019 (UTC)
:::::I don't understand what you mean by "typically". I am interpreting ''clear aperture'' as a synonym for ''entrance pupil'', and as distinct from [[aperture]]. If you are interpreting it differently, what definition are you using? I am starting to regret taking Dicklyon's suggestion—my concern from the beginning was that ''clear aperture'' is an ambiguous term that is not yet defined in the article or elsewhere on Wikipedia, while ''entrance pupil'' is clearly and specifically defined. You write above that "clear aperture" is "the essential concept, the sound and logical foundation". Before it can be a sound foundation for anything, it needs a clear definition.--[[User:Srleffler|Srleffler]] ([[User talk:Srleffler|talk]]) 00:50, 14 August 2019 (UTC)
::::::''Clear aperture'' and ''entrance pupil'' are not synonyms!  When Smith (Modern Optical Engineering, 2nd ed, p142) says, "... clear aperture, or entrance pupil, ..." he is distinguishing the two terms, not equating them.  Smith explains ''clear aperture'' as the clear diameter of a [circular] lens or diaphragm (p133).  That is, whatever central-circular area of a lens or stop which is not obstructed by hardware, defects, imperfections, etc.  This is the essential factor in the definition of relative aperture in terms of a simple lens.  In the case of a multielement system with refraction ahead of the aperture stop, then the image of the aperture stop (the entrance pupil), not the clear aperture, governs.  So the definition is conditional, but still stated in terms of the essential case.  At that point it becomes a philosophical quibble on whether a clear aperture is a degenerate case of a pupil, or a pupil is a glorified case of an aperture, but however you resolve that non-problem, an abstract ratio must be founded on real factors, and that means only clear aperture truly defines relative aperture, with the "or" proviso about the abstract image (pupil) in the multielement condition.  If the WP article doesn't define CA, then it should, instead of referring to the EP that may not apply and is too advanced a concept for an introduction.  This involves the theory of stops (on which Smith has a chapter), and I can testify that expert photographers and even optical engineers don't get that delicacy of a theory.  Smith's textbook is a genuine authority, but it's not perfect on this subject: I once spent hours puzzling over his treatment of "Field lenses and relay systems" described by "CA", which doesn't appear in the index, until I recognized he was abbreviating "clear aperture", also not in the index.  By "typically" I mean in the case of camera lenses, which have entrance pupils differing from the clear aperture, since the article seems to be about camera lens f-numbers and not the theoretical optics. [[User:Rkinch|Richard J Kinch]] ([[User talk:Rkinch|talk]]) 02:51, 14 August 2019 (UTC)
:::::::If they aren't synonyms, then defining f-number in terms of clear aperture is simply wrong. You're defining a general property of an optical system in a way that is meaningful ''only'' for the case of a singlet lens. F-number is widely used for multi-element lens systems. Defining it in a way that doesn't work for such systems makes no sense. The f-number of a camera lens is the ratio of its focal length to the diameter of its entrance pupil. You mention "referring to the EP that may not apply", but the situation is exactly contrary to that: EP applies to ''any'' optical system, while by your definition CA only applies to one special case. 
:::::::The entrance pupil of any optical system (including a singlet lens) is the clear aperture of that system in the sense that it is the largest diameter at which rays entering the system are able to pass through. It is only in this sense that one can say that clear aperture is used to define f-number. --[[User:Srleffler|Srleffler]] ([[User talk:Srleffler|talk]]) 03:39, 14 August 2019 (UTC)
::::::::Some books do treat them as synonyms (not a use I was familiar with, I admit); [https://books.google.com/books?id=g_YXAAAAYAAJ&pg=SA5-PA17&dq=%22clear+aperture%22+pupil&hl=en&sa=X&ved=2ahUKEwiDw9nr4YHkAhXFlFQKHVgyC-4Q6AEwAnoECAAQAg#v=onepage&q=%22clear%20aperture%22%20pupil&f=false], [https://books.google.com/books?id=D0NJOhhjcwsC&pg=PT31&dq=%22clear+aperture%22+pupil&hl=en&sa=X&ved=2ahUKEwiDw9nr4YHkAhXFlFQKHVgyC-4Q6AEwBnoECAQQAg#v=onepage&q=%22clear%20aperture%22%20pupil&f=false].  Is it just in simple telescopes that they talk this way?  I don't know. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 06:50, 14 August 2019 (UTC)
::::::::: The terms are not synonyms because they can't be freely interchanged. Not every clear aperture is an entrance pupil.  That they measure as ''equivalent'' in ''some'' systems (which is what your refs are stating, as a treatment, not a generalization), does not mean that the terms are synonymous.  The term ''clear aperture'' defines relative aperture, because that's the cross-section of a parallel beam transmitted by the system, ''not any object in the system itself''.  Another confusion is that these terms are all referring to the abstract ray geometry, not physical items as is often wrongly assumed.  Beware [[Reification (fallacy)|reification]]. Same reason for "iris" (real object) vs "pupil" (an abstract property).  You can't take a lens apart and point to the pupil, clear aperture, or focal length; these are not physical objects, but abstract properties.  A donut is a physical object, but the outside diameter or hole in the middle is not.  Likewise temperature or heat in thermodynamics, not physical objects (although once misunderstood to be). [[User:Rkinch|Richard J Kinch]] ([[User talk:Rkinch|talk]]) 02:20, 16 August 2019 (UTC)
::::Note that MIL-STD-150A defines relative aperture and f-number in terms of the effective aperture: "The effective aperture of a photographic objective for distant objects, for a given setting of the diaphragm, is an opening equivalent to a right section of the largest beam of parallel light from an axial object point that is transmitted by the lens. It is usually circular, or approximately so, and is specified by its diameter. If the section is not circular, the effective diameter shall be the diameter of a circle having the same equivalent area."  This solves several definition troubles in the WP article: f-number is defined for distant objects on axis, what's the f-number for non-circular pupils, how it applies to a telecentric lens that has no real entrance pupil, etc.  The article should explain how f-number modifies off-axis or at finite conjugates, and specify efl instead of "focal length".  The article is also completely backwards on notation:  f-numbers are a decimal number N, like on a camera lens; the hooked-f form f/N indicates the inverse ''relative aperture'', not f-number.  And 1:N is also seen on lenses. [[User:Rkinch|Richard J Kinch]] ([[User talk:Rkinch|talk]]) 08:14, 15 August 2019 (UTC)
:::::That's a good definition.  The "effective aperture" they describe is exactly the entrance pupil, although maybe it's the case that as you move off-axis the "effective aperture" is a partially occluded entrance pupil.  It would be interesting to find a source that makes such a distinction. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 16:11, 16 August 2019 (UTC)
:::::: ''The'' effective aperture is strictly defined by an axial point source at infinity (parallel rays). Off-axis sources have an aperture that is effective, but that is not the formal characteristic whose singular value is termed "effective aperture". [[User:Rkinch|Richard J Kinch]] ([[User talk:Rkinch|talk]]) 08:18, 20 August 2019 (UTC)
::::::I think Dick is right; for a given diaphragm (aperture stop) setting, the diameter of the largest possible axial bundle of parallel rays is just the diameter of the entrance pupil. --[[User:Srleffler|Srleffler]] ([[User talk:Srleffler|talk]]) 00:33, 24 August 2019 (UTC)
:::::As for the f/N notation, we've been over that many times.  It's sometimes a mathematical expression for diameter, and sometimes just a notation convention for f-number.  More often the latter. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 16:13, 16 August 2019 (UTC)
::::::[[RAS syndrome|The ATM machine reads the PIN number with a camera lens with f-number f/8]]. [[User:Rkinch|Richard J Kinch]] ([[User talk:Rkinch|talk]]) 08:18, 20 August 2019 (UTC)

== photography ==

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. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 01:22, 9 August 2020 (UTC)
:Any of the first three references in the article suffice. Note that it's not a "recent change", but rather a partial revert. The implication that f number is ''only'' used in photography was inserted [https://en.wikipedia.org/w/index.php?title=F-number&type=revision&diff=971775211&oldid=967356156 yesterday]. --[[User:Srleffler|Srleffler]] ([[User talk:Srleffler|talk]]) 16:41, 9 August 2020 (UTC)


== More clarification using the example dealing with theoretical difference between f-numbers.  ==

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.
[[User:Scrutinizer798|Scrutinizer798]] ([[User talk:Scrutinizer798|talk]]) 01:13, 23 September 2020 (UTC)
[[User:Scrutinizer798|Scrutinizer798]] ([[User talk:Scrutinizer798|talk]]) 01:03, 23 September 2020 (UTC)

:I fixed the ambiguity. It was the 100&nbsp;mm {{f/|4}} lens that was intended.
:The fact that the area of a circle is proportional to the square of the diameter is pretty basic, but I added a footnote that references [[Area of a circle]] for readers who are unfamiliar with it. The fact that an opening with four times as much area collects four times as much light is pretty obvious. I don't think it really needs further explanation. --[[User:Srleffler|Srleffler]] ([[User talk:Srleffler|talk]]) 03:02, 24 September 2020 (UTC)

== DSLR and 1/8 stops ==
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.[[Special:Contributions/71.63.160.210|71.63.160.210]] ([[User talk:71.63.160.210|talk]]) 09:01, 27 December 2021 (UTC)
:
:Stepper motors often step smaller than the actual needed step size, so I wouldn't be surprised if they do 1/8. You should have a [[WP:RS]] to put it in the article, though. Otherwise 3/8 is close enough to 1/3. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 11:43, 27 December 2021 (UTC)
::I wonder if that's something I put in there, years ago.  It is something we discovered when I was chief scientist at Foveon, and using Canon lenses.  We wanted to do 1/3-stop increments, but the lenses did 1/8 stop increments.  I doubt I could find an RS for that though.  [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 02:46, 3 December 2022 (UTC)
::Sure enough, that was me, in [https://en.wikipedia.org/w/index.php?title=F-number&diff=50593284&oldid=50539638 this edit in 2006].  Foveon and Sigma had each independently reversed-engineered the Canon EF lens interface, and found that apertures were set by integers representing eighths of stops.  But sorry, I don't know that it was ever published, and I don't know that it's applicable to other interfaces such as Nikon's. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 02:49, 3 December 2022 (UTC)
::Coincidentally, as an irrelevant aside, today I donated to the Computer History Museum a 1999 Foveon Studio Camera ("model 9000" or "Foveon I"), with Canon lens, in completely working condition.  The control, UI, strorage, etc. are all on a Dell Inspiron 7000 series laptop running Windows 98, as part of the camera mounted on the tripod (as [https://digital-photography.org/digital_photography_cameras/Foveon_digital_camera.html here]).  This was the first camera we sold, to Sheldon of Los Altos, in 1999; I recently bought it back for next to nothing.  It's got FoveonCam and FoveonLab 2.7 running nicely (our last release before we pivoted to the X3 sensor technology).  It even has portrait session photos on it still, from their last uses in 2009.  After a decade of Foveon, they moved to a modern DSLR. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 03:25, 3 December 2022 (UTC)
:::
:::3/8 is pretty close to 1/3, probably within uncertainty elsewhere in the system. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 08:02, 3 December 2022 (UTC)
::::Right, but then some of the increments are just 2/8.  Close enough. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 02:26, 4 December 2022 (UTC)
:::::
:::::Yes, but one is one side, and the other the other side, so the difference from an actual 1/3 is pretty small. 1/3-3/8 = 1/24. So each is within about 4% of the marked value. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 05:31, 6 December 2022 (UTC)
::::::We are in agreement.  Perhaps, however, this is just about Canon lenses (and others that emulate them); that's all I know. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 06:57, 6 December 2022 (UTC)
::::
::::Actually, I just checked the old FoveonCam 2.7 user manual, which shows that we supported aperture settings in 1/8 stop increments, denoted like f/5.6 + <small>1/8</small> (that's the example showing in the manual); that was more sensible than trying to make up f-numbers in 1/8-stop increments when the conventional numbers are not that accurate. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 01:55, 6 December 2022 (UTC)

== How close is close? ==

(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. [[User:TooManyFingers|TooManyFingers]] ([[User talk:TooManyFingers|talk]]) 16:14, 19 September 2023 (UTC)
:
:Maybe the article should explain this. The correction factor is 1+M, where M is the magnification. Image size/object size. It is usual, often enough, to use whole stops. (Slide films usually need to do better.)  You would then round when 1+M was about 1.2, or M of 0.2. This is pretty much what everyone calls close-up photography. Among others, there can be  a shadow of the camera or lens on the object.  Most cameras now have through-the-lens metering, so the correction is included. [[User:Gah4|Gah4]] ([[User talk:Gah4|talk]]) 23:40, 19 September 2023 (UTC)