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Talk:F-number

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?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. — 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. 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. 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). 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. this page).  The resulting ratio (in steradians) is inversely proportional to the square of the f-number.   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. 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. BigslyE5 (talk) 15:09, 26 July 2014 (UTC)

And that is why $$F/N=D$$ or focal length divided by f-stop equals Diameter.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. 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 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! 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.  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 Webster, also used in place of entrance pupil in photography. 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. 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.--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. 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. —Ben FrantzDale (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. 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 books. The aperture is the hole; increasing it can only mean making it bigger.  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 (EV, DOF, compensating for vignetting or 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. 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. 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. 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". 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. 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. 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.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. Gah4 (talk) 00:38, 9 April 2019 (UTC)

 * What change are you thinking of, and what sources back it up? 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. 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.)  Gah4 (talk) 04:13, 9 April 2019 (UTC)
 * Just scanning through these book hits and and these.  Dicklyon (talk) 18:50, 9 April 2019 (UTC)
 * The term "f-stop" is actually more commonly used, but has a hard time finding an agreed definition. Here, for example, the f-stop is the reciprocal of the f-number.  More commonly the "f-stop value" or "f-stop measurement" or "f-stop setting" or "f-stop number" is defined like what's commonly called "f-number".  Of these, "f-stop number" is most common, but not nearly as common as "f-number".  Dicklyon (talk) 19:18, 9 April 2019 (UTC)

 * The one you call 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. 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. Dicklyon (talk) 20:59, 9 April 2019 (UTC)

 * f-stop redirects here. aperture has its own article, which duplicates much of this article. 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. 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"?  Dicklyon (talk) 21:58, 9 April 2019 (UTC)

 * Oops, I misinterpreted; that's your impression, not a quote. An opposite impression might come from this optics book, in which f-number is primary and not camera-specific. 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 this book. But also, numerical aperture and has a section Numerical_aperture 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. 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:  "Depth of Field Outside the Box", which will give you an idea on some of the relevant sources. 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.) 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. 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 photographic property.

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.  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. 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. 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. 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)
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)

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. — Preceding unsigned comment added by 50.138.183.186 (talk) 17:33, 9 April 2014 (UTC)

 * 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. Gah4 (talk) 09:45, 4 May 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)
 * 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 sigfig. At extreme close-up distances, even that isn't close. 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. Alfie  ↑↓ © 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. Gah4 (talk) 01:28, 3 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. Gah4 (talk) 01:28, 3 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. 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:
 * 1) 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).
 * 2) 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 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. 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.  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. Gah4 (talk) 17:45, 24 July 2015 (UTC)

I have removed this incorrect parenthetical clause from the article. 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 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)". 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. 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)

 * Well, we should check the appropriate references, but I have always seen the f-number defined as $N = f/D$. There is no contradiction with $N = 1/(2NA)$ because, for any practical lens, $D/f = 2 sin(θ)$.
 * 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, $sin(θ) ≈ tan(θ)$ and the thin lens diagram can be trusted. For fast lenses, $N = f/D = 1/(2 sin(θ))$ still holds as per the Abbe sine condition, but the thin-lens (which implies $D/f = 2 tan(θ)$) is wrong.
 * — Edgar.bonet (talk) 09:44, 4 March 2016 (UTC)
 * Addendum: Here is a derivation of $f/D = 1/(2 sin(θ))$ based on the conservation of etendue. And here is a serious comic discussing why you can't make a lens arbitrarily fast. — 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 $sin(θ)$ and $tan(θ)$ 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. Gah4 (talk) 07:36, 20 July 2016 (UTC)

Numerical_aperture explains tan(θ) vs. sin(θ). Gah4 (talk) 16:18, 13 September 2018 (UTC)

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. 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. — Preceding unsigned comment added by 122.60.105.241 (talk) 23:53, 25 August 2018 (UTC)

 * 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. 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! 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.  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.--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? 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. 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. Dicklyon (talk) 04:34, 12 August 2019 (UTC)

Definition
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)
 * 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.--Srleffler (talk) 03:52, 12 August 2019 (UTC)
 * From p. 183 in Smith's book (emphasis added): "...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..."
 * 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.--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. Richard J Kinch (talk) 22:14, 13 August 2019 (UTC)
 * Something like "clear aperture (entrance pupil) diameter" might be a good compromise? 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". Richard J Kinch (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.--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. Richard J Kinch (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. --Srleffler (talk) 03:39, 14 August 2019 (UTC)
 * Some books do treat them as synonyms (not a use I was familiar with, I admit);, . Is it just in simple telescopes that they talk this way?  I don't know. 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. 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). Richard J Kinch (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. Richard J Kinch (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. 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". Richard J Kinch (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. --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. Dicklyon (talk) 16:13, 16 August 2019 (UTC)
 * The ATM machine reads the PIN number with a camera lens with f-number f/8. Richard J Kinch (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. 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 yesterday. --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. Scrutinizer798 (talk) 01:13, 23 September 2020 (UTC) Scrutinizer798 (talk) 01:03, 23 September 2020 (UTC)

 * I fixed the ambiguity. It was the 100 mm 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. --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.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. 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.  Dicklyon (talk) 02:46, 3 December 2022 (UTC)
 * Sure enough, that was me, in 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. 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 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. Dicklyon (talk) 03:25, 3 December 2022 (UTC)
 * 3/8 is pretty close to 1/3, probably within uncertainty elsewhere in the system. Gah4 (talk) 08:02, 3 December 2022 (UTC)
 * Right, but then some of the increments are just 2/8. Close enough. 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. 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. 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 + 1/8 (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. Dicklyon (talk) 01:55, 6 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. 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. 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 + 1/8 (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. Dicklyon (talk) 01:55, 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 + 1/8 (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. 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. 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. Gah4 (talk) 23:40, 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. Gah4 (talk) 23:40, 19 September 2023 (UTC)

title: Talk:F-number


text:
Edwin Fischer

Edwin Fischer (6 October 1886 – 24 January 1960) was a Swiss classical pianist and conductor. He is regarded as one of the great interpreters of J.S. Bach and Mozart in the twentieth century.

Biography
Fischer was born in Basel and studied music first there with Hans Huber, and later in Berlin at the Stern conservatory under Martin Krause. He first came to prominence as a pianist following World War I. In 1926, he became conductor of the Lübeck Musikverein and later conducted in Munich. In 1932, he formed his own chamber orchestra, and was one of the first to be interested in presenting music of the Baroque and Classical periods in a historically accurate way. Although his performances were not historically accurate by present-day standards, they were for his time; e.g., he did conduct Bach and Mozart concertos from the keyboard, an unusual practice at that time. In 1932, he returned once again to Berlin, succeeding Artur Schnabel in a teaching role at the Berlin Hochschule für Musik, who had emigrated from Germany because of increasing anti-Semitism. In 1942, he moved back to Switzerland, temporarily putting his career on hold through World War II. After the war he began to perform again, and gave master classes in Lucerne for a number of later prominent pianists such as Alfred Brendel, Helena Sá e Costa, Mario Feninger, Reine Gianoli, Paul Badura-Skoda and Daniel Barenboim. As well as solo recitals, concerto performances and conducting, Fischer performed much chamber music. Particularly highly regarded was the piano trio he formed with the cellist Enrico Mainardi and the violinist Georg Kulenkampff (who was replaced by Wolfgang Schneiderhan after Kulenkampff's death). Fischer published a number of books on teaching, and one on the piano sonatas of Beethoven. He also made a number of recordings, including the first complete recording of Bach's Well-Tempered Clavier for EMI, recorded on the piano from 1933 to 1936. Fischer's historically important recordings have been reissued on CD by EMI and specialty labels such as APR, Music & Arts, Pearl and Testament. Other classic recordings by Fischer include Bach keyboard concertos, miscellaneous solo Bach works such as the Chromatic Fantasia and Fugue, various sonatas by Mozart and Beethoven, Schubert's Wanderer Fantasy and Impromptus, Beethoven's "Emperor" and Brahms' Second Piano Concerto, both conducted by Wilhelm Furtwängler, and Mozart concertos K. 453, 466, 482, 491 and 503, the last three conducted by John Barbirolli, Lawrance Collingwood and Josef Krips in 1935, 1937 and 1947 respectively.

Fischer also accompanied Elisabeth Schwarzkopf in a renowned 1950s EMI LP of Schubert Lieder. His last musical collaboration was with the violinist Gioconda de Vito. During their recording sessions for the Brahms first and third violin sonatas, he had to go to London for medical treatment, where he was told he was seriously ill. He died shortly afterwards in Zürich. His mortal remains were cremated and his ashes buried at the Friedental cemetery in Lucerne.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Edwin Fischer


text:
Paul Butterfield

Paul Vaughn Butterfield (December 17, 1942 – May 4, 1987) was an American blues harmonica player, singer and bandleader. After early training as a classical flautist, he developed an interest in blues harmonica. He explored the blues scene in his native Chicago, where he met Muddy Waters and other blues greats, who provided encouragement and opportunities for him to join in jam sessions. He soon began performing with fellow blues enthusiasts Nick Gravenites and Elvin Bishop.

In 1963, he formed the Paul Butterfield Blues Band, which recorded several successful albums and was popular on the late-1960s concert and festival circuit, with performances at the Fillmore West in San Francisco, the Fillmore East in New York City, the Monterey Pop Festival, and Woodstock. The band was known for combining electric Chicago blues with a rock urgency and for their pioneering jazz fusion performances and recordings. The band was also among the first racially integrated blues groups. After the breakup of the group in 1971, Butterfield continued to tour and record with the band Paul Butterfield's Better Days, with his mentor Muddy Waters, and with members of the roots-rock group the Band. While still recording and performing, Butterfield died in 1987 at age 44 of an accidental drug overdose.

Music critics have acknowledged his development of an original approach that places him among the best-known blues harp players. In 2006, he was inducted into the Blues Hall of Fame. Butterfield and the early members of the Paul Butterfield Blues Band were inducted into the Rock and Roll Hall of Fame in 2015. Both panels noted his harmonica skills and his contributions to bringing blues music to a younger and broader audience.

Career
Paul Vaughn Butterfield was born on December 17, 1942 in Chicago and raised in the city's Hyde Park neighborhood. The son of a lawyer and a painter, he attended the University of Chicago Laboratory Schools, a private school associated with the University of Chicago. Exposed to music at an early age, he studied classical flute with Walfrid Kujala, of the Chicago Symphony Orchestra. Butterfield was also athletic and was offered a track scholarship to Brown University. However, a knee injury and a growing interest in blues music sent him in a different direction. He met guitarist and singer songwriter Nick Gravenites, who shared an interest in authentic blues music. By the late 1950s, they were visiting blues clubs in Chicago, where musicians such as Muddy Waters, Howlin' Wolf, Little Walter, and Otis Rush encouraged them and occasionally let them sit in on jam sessions. The pair were soon performing as Nick and Paul in college-area coffee houses.

In the early 1960s, Butterfield met aspiring blues guitarist Elvin Bishop. Bishop recalled: "He [Butterfield] was playing more guitar than harp when I first met him. But in about six months he became serious about the harp, and he seemed to get about as good as he got in that six months. He was just a natural genius. This was in 1960 or 1961.  By this time Butter had been hanging out in the ghetto for a couple of years, and he was part of the scene and getting accepted." Eventually, Butterfield, on vocals and harmonica, and Bishop, accompanying him on guitar, were offered a regular gig at Big John's, a folk club in the Old Town district on Chicago's near North Side. With this booking, they persuaded bassist Jerome Arnold and drummer Sam Lay (both from Howlin' Wolf's touring band) to form a group with them in 1963. Their engagement at the club was highly successful and brought the group to the attention of record producer Paul A. Rothchild.

Paul Butterfield Blues Band
During their engagement at Big John's, Butterfield met and occasionally sat in with guitarist Mike Bloomfield, who was also playing at the club. By chance, producer Rothchild witnessed one of their performances and was impressed by the chemistry between the two. He persuaded Butterfield to bring Bloomfield into the band, and they were signed to Elektra Records. Their first attempt to record an album, in December 1964, did not meet Rothchild's expectations. To capture their sound better, Rothchild convinced Elektra president Jac Holzman to record a live album. In the spring of 1965, the Butterfield Blues Band was recorded at the Cafe Au Go Go in New York City. These recordings also failed to satisfy Rothchild, but the group's appearances at the club brought them to the attention of the East Coast music community. Rothchild persuaded Holzman to agree to a third attempt at recording an album., securing the band more engagements outside of Chicago. At the last minute, the band was booked to perform at the Newport Folk Festival in July 1965. Maria Muldaur, with her husband Geoff, who later toured and recorded with Butterfield, recalled the group's performance as stunning; it was the first time that many of the mostly folk-music fans had heard a high-powered electric blues combo. Among those who took notice was festival regular Bob Dylan, who invited the band to back him for his first live electric performance. With little rehearsal, Dylan performed a short, four-song set the next day with Bloomfield, Arnold, and Lay (along with Al Kooper and Barry Goldberg). The performance was not well received by some and generated a controversy, but it was a watershed event and brought the band to the attention of a much larger audience.

The band's debut album, The Paul Butterfield Blues Band, was released in 1965, reaching number 123 in the Billboard 200 album chart. On March 28, 1966, Butterfield appeared on the CBS game show To Tell the Truth. At the end of his segment, he performed "Born in Chicago" with the house band. In August 1966, the band released their second album, East-West, which reached number 65 in the album chart.

In England in November 1966, Butterfield recorded several songs with John Mayall & the Bluesbreakers, who had recently finished the album A Hard Road. Butterfield and Mayall contributed vocals, and Butterfield's Chicago-style blues harp was featured. Four songs were released in the UK on a 45-rpm EP, John Mayall's Bluesbreakers with Paul Butterfield, in January 1967.

The Butterfield Blues Band changed its lineup, with Arnold and Davenport leaving the band, and Bloomfield going on to form his own group, Electric Flag. The band added bassist Bugsy Maugh, drummer Phillip Wilson, and saxophonists David Sanborn and Gene Dinwiddie. This lineup recorded the band's third album, The Resurrection of Pigboy Crabshaw, in 1967. It was Butterfield's highest-charting album, reaching number 52 on the album chart. Most of this lineup performed at the seminal Monterey Pop Festival on June 17, 1967.

The band's next album, In My Own Dream, released in 1968, continued to move away from the band's Chicago blues roots towards a more soul-influenced, horn-based sound, with Butterfield singing only three songs. It reached number 79 in the Billboard album chart. By the end of 1968, both Bishop and Naftalin had left the band. In April 1969, Butterfield took part in a concert at Chicago's Auditorium Theater and a subsequent recording session organized by record producer Norman Dayron, featuring Muddy Waters backed by Otis Spann, Mike Bloomfield, Sam Lay, Donald "Duck" Dunn, and Buddy Miles. Such Waters warhorses as "Forty Days and Forty Nights", "I'm Ready", "Baby, Please Don't Go", and "Got My Mojo Working" were recorded and later released on the album Fathers and Sons. Waters commented, "We did a lot of the things over we did with Little Walter and Jimmy Rogers and Elgin Evans on drums [an early configuration of Waters's band] ... It's about as close as I've been [to that feel] since I first recorded it". To one reviewer, these recordings represent Paul Butterfield's best performances.

The Butterfield Blues Band was invited to perform at the Woodstock Festival on August 18, 1969. The band performed seven songs, and although its performance did not appear in the film Woodstock, one song, "Love March", was included on the album Woodstock: Music from the Original Soundtrack and More, released in 1970. In 2009, Butterfield was included in the expanded 40th Anniversary Edition Woodstock video, and an additional two songs appeared on the box set Woodstock: 40 Years On: Back to Yasgur's Farm.

The album Keep On Moving, with only Butterfield remaining from the original lineup, was released in 1969. It was produced by veteran R&B producer and songwriter Jerry Ragovoy, reportedly brought in by Elektra to turn out a "breakout commercial hit". The album was not embraced by critics or long-time fans. It reached number 102 in the Billboard album chart.

A live double album by the Butterfield Blues Band, Live, was recorded March 21–22, 1970, at The Troubadour, in West Hollywood, California. By this time, the band included a four-piece horn section in what has been described as a "big-band Chicago blues with a jazz base". After the release of another soul-influenced album, Sometimes I Just Feel Like Smilin' in 1971, the Paul Butterfield Blues Band disbanded. In 1972, a retrospective of their career, Golden Butter: The Best of the Paul Butterfield Blues Band, was released by Elektra.

Better Days and solo
After the breakup of the Butterfield Blues Band and no longer under contract with Elektra, Butterfield retreated to Woodstock, New York, where he eventually formed his next band, Paul Butterfield's Better Days, with drummer Chris Parker, guitarist Amos Garrett, singer Geoff Muldaur, pianist Ronnie Barron and bassist Billy Rich. In 1972–1973, the group recorded the albums Paul Butterfield's Better Days and It All Comes Back, released by Albert Grossman's Bearsville Records. The albums reflected the influence of the participants and explored more roots- and folk-based styles. Although without an easily defined commercial style, both reached the album chart. The band did not last to record a third studio album, but its album Live at Winterland Ballroom, recorded in 1973, was released in 1999.

Butterfield next pursued a solo career and appeared as a sideman in several different musical settings. In 1975, he again joined Muddy Waters to record Waters's last album for Chess Records, The Muddy Waters Woodstock Album. The album was recorded at Levon Helm's Woodstock studio with Garth Hudson and members of Waters's touring band. In 1976, Butterfield performed at the Band's final concert, "The Last Waltz", accompanying the Band on the song "Mystery Train" and backing Muddy Waters on "Mannish Boy". Butterfield kept up his association with former members of the Band, touring and recording with Levon Helm and the RCO All Stars in 1977 and touring with Rick Danko in 1979. A 1984 live performance with Danko and Richard Manuel was recorded and released as Live at the Lonestar in 2011.

As a solo act with backing musicians, Butterfield continued to tour and recorded Put It in Your Ear in 1976 and North South in 1981, with strings, synthesizers, and funk arrangements. In 1986, he released his final studio album, The Legendary Paul Butterfield Rides Again, which was an attempt at a comeback with an updated rock sound. On April 15, 1987, he participated in the concert "B.B. King & Friends", with Eric Clapton, Etta James, Albert King, Stevie Ray Vaughan, and others.

Legacy
Aside from "rank[ing] among the most influential harp players in the Blues", Butterfield has also been seen as pointing blues-based music in new, innovative directions. AllMusic critic Steve Huey commented, "It's impossible to overestimate the importance of the doors Butterfield opened: before he came to prominence, white American musicians treated the blues with cautious respect, afraid of coming off as inauthentic. Not only did Butterfield clear the way for white musicians to build upon blues tradition (instead of merely replicating it), but his storming sound was a major catalyst in bringing electric Chicago blues to white audiences who'd previously considered acoustic Delta blues the only really genuine article."

In 2006, Butterfield was inducted into the Blues Foundation's Blues Hall of Fame, which noted that "the albums released by the Butterfield Blues Band brought Chicago Blues to a generation of Rock fans during the 1960s and paved the way for late 1960s electric groups like Cream". The Rock and Roll Hall of Fame inducted the Paul Butterfield Blues Band in 2015. The induction biography commented that "the Butterfield Band converted the country-blues purists and turned on the Fillmore generation to the pleasures of Muddy Waters, Howlin' Wolf, Little Walter, Willie Dixon and Elmore James".

In 2017, a documentary titled Horn from the Heart: the Paul Butterfield Story premiered at the Newport Beach Film Festival. Directed by John Anderson and produced by Sandra Warren, it won the Outstanding Achievement Award in Filmmaking: Editing. In October 2018, the documentary was released nationally in select US theaters. It has received critical acclaim, including being named a New York Times Critic's Pick, as well as features in Rolling Stone, and The Wall Street Journal.

Harmonica style
Like many Chicago blues harp players, Butterfield approached the instrument like a horn, preferring single notes to chords, and used it for soloing. His style has been described as "always intense, understated, concise, and serious", and he was "known for purity and intensity of his tone, his sustained breath control, and his unique ability to bend notes to his will". In his choice of notes he has been compared to Big Walter Horton, but he was never seen as an imitator of any particular harp player. Rather, he developed "a style original and powerful enough to place him in the pantheon of true blues greats".

Butterfield played Hohner harmonicas (and endorsed them). He preferred the diatonic ten-hole Marine Band model. He wrote a harmonica instruction book, Paul Butterfield Teaches Blues Harmonica Master Class, a few years before his death (it was not published until 1997). In it, he explains various techniques, demonstrated on an accompanying CD. Butterfield played mainly in cross-harp, or second position. Reportedly left-handed, he held the harmonica in a manner opposite that of a right-handed player, i.e., in his right hand, upside down (with the low notes to the right), using his left hand for muting effects.

Also like other electric Chicago blues harp players, Butterfield frequently used amplification to achieve his sound. He has been associated with a Shure 545 Unidyne microphone, although producer Rothchild noted that around the time of a 1965 recording session, Butterfield favored an Altec harp microphone run through an early model Fender tweed amplifier. Beginning with album The Resurrection of Pigboy Crabshaw, he used an acoustic harmonica style, following his shift to a more R&B-based approach.

Personal life
By all accounts, Paul Butterfield was absorbed in his music. According to his brother Peter, "He listened to records and went places, but he also spent an awful lot of time, by himself, playing [harmonica]. He'd play outdoors. There's a place called the Point in Hyde Park [Chicago], a promontory of land that sticks out into Lake Michigan, and I can remember him out there for hours playing. He was just playing all the time ... It was a very solitary effort. It was all internal, like he had a particular sound he wanted to get and he just worked to get it." Producer Norman Dayron recalled the young Butterfield as "very quiet and defensive and hard-edged. He was this tough Irish Catholic, kind of a hard guy. He would walk around in black shirts and sunglasses, dark shades and dark jackets ... Paul was hard to be friends with." Although they later became close, Michael Bloomfield commented on his first impressions of Butterfield: "He was a bad guy. He carried pistols. He was down there on the South Side, holding his own. I was scared to death of that cat". Writer and AllMusic founder Michael Erlewine, who knew Butterfield early in his recording career, described him as "always intense, somewhat remote, and even, on occasion, downright unfriendly". He remembered Butterfield as "not much interested in other people".

By 1971, Butterfield had purchased his first house, in rural Woodstock, New York, and began enjoying family life with his second wife, Kathy Peterson, and their infant son, Lee. According to Maria Muldaur, she and her husband were frequent dinner guests, which usually involved sitting around a piano and singing songs. She doubted her abilities, but "it was Butter that [sic] first encouraged me to let loose and just sing the blues [and] not to worry about singing pretty or hitting all the right notes ... He loosened all the levels of self-consciousness and doubt out of me ... And he'll forever live in my heart for that and for respecting me as a fellow musician."

Death
Beginning in 1980, Butterfield underwent several surgical procedures to relieve his peritonitis, a serious and painful inflammation of the intestines. Although strongly opposed to heroin as a bandleader, he developed an addiction to it, which, according to Steve Huey in AllMusic's Butterfield biography, led to "speculation that he was trying to ease his peritonitis symptoms". The financial strain of supporting his drug habit was bankrupting him, and the deaths of his friend and one-time musical partner Mike Bloomfield, and manager Albert Grossman had shaken him. On May 4, 1987, at age 44, Butterfield died at his apartment in the North Hollywood district of Los Angeles. An autopsy by the county coroner concluded that he was the victim of an accidental drug overdose, with "significant levels of morphine (heroin), ... codeine, the tranquilizer Librium and a trace of alcohol."

By the time of his death, Butterfield was out of the commercial mainstream. Although for some, he was very much the blues man, Maria Muldaur commented "he had the whole sensibility and musicality and approach down pat ... He just went for it and took it all in, and he embodied the essence of what the blues was all about. Unfortunately, he lived that way a little too much."

Discography
In 1964, Butterfield began his association with Elektra Records and eventually recorded seven albums for the label. After the break up of the Butterfield Blues Band in 1971, he recorded four albums for manager Albert Grossman's Bearsville Records – two with Paul Butterfield's Better Days and two solo. His last solo album was released by Amherst Records. After his death in 1987, his former record companies released a number of live albums and compilations.

The Butterfield Blues Band

 * The Paul Butterfield Blues Band (1965) (peaked at number 123 on the Billboard 200 album chart)
 * East-West (1966) (No. 65 on Billboard 200)
 * The Resurrection of Pigboy Crabshaw (1967) (No. 52 on Billboard 200)
 * In My Own Dream (1968) (No. 79 on Billboard 200)
 * Keep On Moving (1969) (No. 102 on Billboard 200)
 * Sometimes I Just Feel Like Smilin' (1971) (No. 124 on Billboard 200)

Paul Butterfield's Better Days

 * Better Days (1973) (No. 145 on Billboard 200)
 * It All Comes Back (1973) (No. 156 on Billboard 200)

Paul Butterfield

 * Put It in Your Ear (1976)
 * North–South (1981)
 * The Legendary Paul Butterfield Rides Again (1986)

Live albums
All by the Paul Butterfield Blues Band, except as noted.
 * Live (1970)[2LP] – reissued 2005 on CD with bonus tracks (Billboard 200 No. 72)
 * Strawberry Jam (1996) – rec. 1966–68
 * East-West Live (1996) – rec. 1966–67
 * Live at Unicorn Coffee House – rec. 1966. released with several titles and dates, bootleg.
 * Paul Butterfield's Better Days, Live at Winterland Ballroom (1999) – rec. 1973
 * Paul Butterfield Band, Rockpalast: Blues Rock Legends, Vol. 2 (2008) – rec. 1978
 * Rick Danko, Richard Manuel & Paul Butterfield, Live at the Lone Star, (2011) – rec. 1984
 * Live in White Lake, N.Y. 8/18/69 (2015) – released as part of The Complete Albums 1965–1980
 * Live at Woodstock (2020)[2LP]

Butterfield compilation albums

 * Golden Butter: The Best of the Paul Butterfield Blues Band (1972)[2LP] – Billboard 200 No. 136
 * The Original Lost Elektra Sessions (1995) – rec. 1964
 * An Anthology: The Elektra Years (1997)[2CD]
 * Paul Butterfield's Better Days, Paul Butterfield's Better Days: Bearsville Anthology (2000)
 * Hi-Five: The Paul Butterfield Blues Band (2006) – EP

Compilation albums and videos with various artists

 * Folksong '65 (1965)
 * What's Shakin' (1966)
 * Festival (1967 film, including 1965 appearance with Dylan)
 * You Are What You Eat (1968 film soundtrack)
 * Woodstock: Music from the Original Soundtrack and More (1970, recorded 1969)
 * Woodstock 2 (1971, recorded 1969)
 * An Offer You Can't Refuse (1972, recorded 1963)
 * Woodstock '79 (1991 video, filmed 1979)
 * Woodstock: Three Days of Peace and Music (1994, recorded 1969)
 * The Monterey International Pop Festival June 16–17–18 30th Anniversary Box Set (1997, recorded 1967)
 * The Complete Monterey Pop Festival (2002 video, filmed 1967)
 * Woodstock: 40 Years On: Back to Yasgur's Farm (2009, recorded 1969)
 * Woodstock: 40th Anniversary Ultimate Collector's Edition (2009 video, filmed 1969)

As accompanist

 * The Peter, Paul and Mary Album (1966), Peter, Paul and Mary, "The King of Names" with Butterfield, Bloomfield, and Naftalin
 * John Mayall's Bluesbreakers with Paul Butterfield (1967 EP), John Mayall & the Bluesbreakers
 * Blues at Midnight (released with several titles and dates), Jimi Hendrix, B.B. King, and others (bootleg of jam recorded 1968)
 * Fathers and Sons (1969, reissued 2001 with bonus tracks), Muddy Waters
 * Give It Up (1972), Bonnie Raitt
 * 2nd Right, 3rd Row (1972), Eric Von Schmidt
 * Steelyard Blues (1973 film soundtrack), Mike Bloomfield, Nick Gravenites, Maria Muldaur, and others
 * That's Enough for Me (1973), Peter Yarrow
 * The Muddy Waters Woodstock Album (1975), Muddy Waters
 * Levon Helm & the RCO All-Stars (1977)
 * Already Free (1977), Nick Jameson
 * The Last Waltz (1978), the Band
 * Elizabeth Barraclough (1978), Elizabeth Barraclough
 * Hi! (1979), Elizabeth Barraclough
 * Down by Law (1985), Deadline
 * B.B. King & Friends (released with various titles and dates), B.B King, Eric Clapton, Stevie Ray Vaughan, and others (bootleg video of television special filmed 1987)
 * Heart Attack (1990, recorded 1986), Little Mike & the Tornados

Tribute albums

 * A Tribute to Paul Butterfield, Robben Ford and the Ford Blues Band (2001)
 * The Butterfield/Bloomfield Concert, the Ford Blues Band, with Robben Ford and Chris Cain (2006)

title: Paul Butterfield