Talk:Circle of confusion

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I am confused to :) Depth of field range of distances in which the image is in focus. Depth of field depends of an admissible circle of confusion diameter. There is no objective way to define this diameter.

Can you help me to turn this mess in good English.

Seems like a reference to digital-camera sensor pixel size would be useful here — is it true that if you want to make use of your sensor's full resolution, you need to keep the COC below the size of a pixel? For example, a Canon EOS-20D has a pixel size on the sensor of approximately .064 mm by my calculation. Steve 03:14, 7 April 2006 (UTC)

Steve, that issue has been addressed by Doug Kerr here: [1] in which he compared the visual acuity outlook with the camera resolution outlook. This latter outlook has not been widely accepted, but you can write about it here if you like. I think you slipped a digit in your pixel size calculation, and you might want to choose a COC bigger than that anyway, consistent with the anti-aliasing filter spot size more than the pixel size. Dicklyon 04:26, 7 April 2006 (UTC)

You're right, I did slip a digit, the 20D has a .0064mm pixel size. Thanks. And thanks for the info. Steve 16:07, 7 April 2006 (UTC)

[edit] Delete Circle of confusion computation?

I've listed Circle of confusion computation for deletion. Please take a look if you care, and leave a comment. Dicklyon 04:22, 11 October 2006 (UTC)

I merged it, with a rewrite and new figure. Dicklyon 05:23, 23 October 2006 (UTC)

Dick, I think the dots should be eliminated from the equations (see Wikipedia:Manual of Style (mathematics)#Multiplication_sign). The MoS seems in accord with most other practice. JeffConrad 07:17, 23 October 2006 (UTC)
Jeff, I correctly anticipated that you would say that. I decided to leave it to you to try to make them look good without dots, because I was having trouble. I don't interpret that MoS entry as saying that the middot is not an allowable option. Dicklyon 15:27, 23 October 2006 (UTC)

[edit] Other edits of 23&ndash24 October 2006

I've tried to simplify and clarify the criteria for choosing a circle-of-confusion diameter limit, which I've termed "CoC" in light of the most common meaning among non–optical designers. I've never heard of 50 cm as the assumed viewing distance (was that really what was intended?), so I replaced it with the nearly universal 25 cm. Moreover, I've never seen a reference suggesting that the final-image blur threshold had anything to do with other than the viewing distance, so I eliminated mention of the final-image diagonal in that context (presumably, the intended reference was to enlargement of the original image).

I think it said 30 cm, not 50. That was among the early conventions (12 inches was, anyway, like the "12 to 18 inches" in the T. H. article). But you're right that 25 cm is more conventional.
I disagree about the final image size not being accepted as a relevant parameter for modifying viewing distance. I have researched this history extensively, the it's all over as one of two alternatives on how CoC should be set. Using a fixed viewing distance and a print size that makes the perspective "correct" is one alternative. The other is to assume a viewing distance that's appropriate to the print size, approximately equal to the image diagonal. Neither of these ignores print size, which is a third alternative. See my draft paper section on "Choosing the COC Criterion" for a rambling discussion and references. Dicklyon 15:25, 23 October 2006 (UTC)
My reference of 25 cm admittedly takes no note of historical assumptions about visual acuity (though at 30 cycles/degree, Snellen's criteria of 100 years ago really don't differ that much from current values). I didn't mean to suggest that final-image size isn't a relevant parameter for modifying viewing distance, but simply that image size doesn't directly affect visual acuity. It's a two-step process: 1) the viewing distance may be dictated by the final-image size, and 2) the final-image CoC is determined by the viewing distance. Let's face—hardly anyone views an image at the "perspective-correct" distance, especially when that distance would be less than about 25 cm. There is considerable argument for viewing an image larger than 8×10 at a distance greater than 25 cm, unless the viewer is David Hemmings or simply is looking to find imperfections rather than to appreciate the image (if you're looking for trouble, it's usually not hard to find). There seems to be one school of thought that critical viewing of large prints requires a very small CoC (e.g., 0.010 mm for full-frame 35 mm format). There are at least two problems with this approach, however: 1) motion blur often is a problem, and 2) diffraction defeats the entire purpose at all but the shallowest required DoF. I discuss this in some detail, under "Diffraction," in my DoF paper (PDF). The conclusion, in essence, is that conventional CoC criteria aren't all that far from the mark. JeffConrad 00:31, 24 October 2006 (UTC)
Ah, yes, but which convention? There's the rub. My DoF paper also talks about the diffraction limit, esp. for macro shooting where it is always a first-order effect to trade off with DoF. Dicklyon 00:45, 24 October 2006 (UTC)
Criteria based on detectable blur threshold, and derived from angular visual acuity, and assuming some reasonable viewing conditions; one could quibble about specific values (as many have ...), but the common assumption of an 8×10 final image viewed at 25 cm (and correspondingly greater distances for larger images) seems a reasonable fit to both necessary and practically achievable conditions. There always will be people who insist on viewing very large images at close distances (I'll admit to having done so myself), but it simply may not be possible to ensure that such people always see an acceptably sharp image. For example, Canon's purported CoC of 0.035 mm (for full-frame 35 mm) has sometimes been criticized as a bit loose. In various editions of Lens Work, Canon state that this value derives from the assumption of 5× enlargement, so it really is in accord with common assumptions about visual acuity and viewing. For an 8×10 enlargement, the common assumptions stated above would dictate a CoC of 0.025 mm, which is easily accomplished by closing down one step from the f-number indicated by the lens DoF scale (or the Depth-of-Field AE mode on some cameras). The attendant increase in exposure time my pose a minor problem, but diffraction isn't an issue unless great DoF is required. The process cannot continue forever, however. There always is a loss of sharpness in the plane of focus once a lens is stopped down a step or so beyond f-number at which aberrations are well corrected, but eventually, further stopping down decreases sharpness even at the DoF limits. With an extremely small CoC (e.g., 0.010 mm) sometimes suggested for "critical" viewing, diffraction becomes the limiting factor for almost any DoF that extends beyond the plane of focus. Similar values for CoC have been suggested in order to capture all detail of which the film (or whatever ...) is capable; however, imaging media are pretty much independent of format, so this approach would use a CoC of 0.010 mm for 8×10 as well (1/20th the common value of 0.2 mm), an obviously unworkable situation.
Simply stated, it just is not possible to have an image with DoF acceptably sharp under any arbitrary viewing conditions. DoF/CoC criteria that derive from assuming an 8×10 final image viewed at 25 cm represent minimum necessary, but they also are not far from what often are the maximum practical. It's possible to reduce these values slightly, but diffraction quickly makes the process self-defeating. JeffConrad 02:12, 24 October 2006 (UTC)
One problem with the whole 8x10 idea is what to do about the 2:3 aspect ratio of 35mm cameras. Use 8x12? or 6.67x10? It's a 20% effect on your choice of CoC. Using diagonals helps, and I think that's why they've been popular. 0.2 mm on 8x12 is d/1830, but on 6.67x10 it's 1/1530, more in line with typical usage I think.

It would seem clear that in most photographic contexts, terms such as "film," "negative," and "print" are now unreasonably restrictive (and there soon may be readers who never have heard of film). I've used "original image" in place of negative, and "final image" in place of print. I don't have especially strong feelings about either term, but it would seem sensible to adopt consistent terminology for which the meanings are reasonably self-evident. JeffConrad 07:32, 23 October 2006 (UTC)

I made a few more edits about that; putting film and print parenthetically to make sure the new terms are understood in terms of the old, in case they're not self-evident to everybody; and changing another film reference to original image. And I reversed the old point that was there about CoC needing to be smaller if the reproduction is blurry, and explained why the opposite is more true. Dicklyon 15:25, 23 October 2006 (UTC)
Doug Pardee's references to "capture" under Limitations of DOF formulae in the Depth of field article suggest "captured image" as an alternative to "original image"; I've used the term myself in a few papers. The role of Wikipedia editors isn't really to create new terms, but some alternative to "film" and similar clearly is needed, and absent consensus terminology in one or more "verifiable" sources, some initiative may be needed here. At the very least, consistency among related articles would seem helpful to the reader. Any thoughts? JeffConrad 01:04, 24 October 2006 (UTC)
I never much cared for the "capture" concept; too vague. But I haven't checked the literature to see if there's a good precedent here. Dicklyon 02:47, 24 October 2006 (UTC)

I still have a bit of a problem with 'The CoC of d/1500 is intended to represent "average" reproduction and viewing conditions.' I don't dispute that it's in common use, but I don't know what "average" means here. There have been more criteria for CoC than I can count, including f/1000 (deprecated for some time), diagonal/1500, diagonal/1730, and many others. I don't have any great ideas for rewording, though, so I've left it alone. JeffConrad 00:31, 24 October 2006 (UTC)

Yes, I agree. I hope I'm not the one who wrote "average", but it's possible. I'll think about it. By the way, the d/1730 never was used by anybody, as far as I can tell, with the exception of an online calculator and a community of people who were misled by web nonsense. Dicklyon 00:45, 24 October 2006 (UTC)
You are preaching to the converted ... Knowledge for the people; give them a light and they will follow it anywhere. It's particularly true for net lemmings (witness "light value"). The only safe bet is to treat anything that doesn't cite a verifiable source as urban legend. JeffConrad 02:23, 24 October 2006 (UTC)
I found this diff in which the quoted "average" came in, way back in '04. Its sentence has evolved over time, and I should have flushed it when I had a chance. My usual way of saying these things is that it's a "conventional" choice based a "typical" condition or something like that. You had backed out one of my "conventional" lines, so now I wonder if that term seems too vague for you. That diff is also where the quotationed "Zeiss formula" came in, though it was a red link for a long time until I wrote the article on it. Dicklyon 02:47, 24 October 2006 (UTC)
There were quite a few "conventionals" in the same sentence ... I just tried to make it more concise; I didn't think my edit really changed the meaning. Strictly speaking, of course, it's one thing to note that a practice is common, but another to determine the basis without a solid history of its usage. Perhaps it suffices to note the practice without attempting to infer the basis (which might be the same for d/1500 as for d/1730 ...). Again, I like criteria that derive from considerations of visual acuity and viewing, recognizing that they don't give the entire picture. JeffConrad 03:05, 24 October 2006 (UTC)

Yet another attempt. A bit general, perhaps, but it avoids assumptions about "average" or "typical." What really matters is the difference between actual reproduction and viewing conditions and those assumed in determining the CoC, which aren't always d/1500 (for example, Canon claim an a 0.035 mm CoC for full-rame 35 mm cameras). I think the "common" assumptions have been stated more than adequately in the preceding paragraphs.

I deleted the reference to apparent DoF because all DoF is apparent by definition. JeffConrad 02:00, 25 October 2006 (UTC)

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