For digital sensors and in terms of imaging medium, is the minimum CoC equal to the size of 1 sensor pixel or 2? And why? For example purposes using a 35mm Full Frame digital sensor where the pixel size is 0.00639mm (and rounding up), is the minimum CoC 0.007mm or 0.014mm?
In What exactly determines depth of field? jrista says
Digital sensors do have a fixed minimum size for CoC, as the size of a single sensel is as small as any single point of light can get (in a Bayer sensor, the size of a quartet of sensels is actually the smallest resolution.)
However, later on jrista says
In the average case, one can assume that CoC is always the minimum achievable with a digital sensor, which these days rolls in at an average of 0.021mm, although a realistic range covering APS-C, APS-H, and Full Frame sensors covers anywhere from 0.015mm - 0.029mm
Using the number 0.015mm for minimum CoC on digital full frame sensors is about 2 sensor pixels in size instead of 1, does this not match up to what was said originally? Or does it, by implying (but not explicitly stating) the use of a bayer sensor which is said above to have a minimum CoC equal to a quartet of pixels, and that would be 2 pixels wide and ~0.0015mm?
And in Why do some people say to use 0.007 mm (approximate pixel size) for the CoC on a Canon 5DM2? Michael Clark says
With digital sensors, the size of the pixel determines the size at which the circle of confusion (CoC) becomes significant when viewing at 100% crops. Any blur circle smaller than the pixel pitch will be recorded as a single pixel. Only when the blur circle becomes larger than an individual pixel will it be recorded by two adjacent pixels.
However, this webpage that is very interesting says
The smallest size of the image CoC may be limited by other factors. For digital sensors, the CoC cannot be smaller than the physical size of two pixels (image elements). Obviously nothing smaller can be resolved. Typical pixel sizes for high resolution digital cameras are in the range of .006 to .012mm. These sizes yield resolution numbers of 83 lp/mm and 43 lp/mm respectively. These equate to CoC values of .012 and .023mm. A similar effect is unavoidable with film emulsions since the grain size determines the size of an individual image element. The typical “graininess” of film varies from .004 to .018mm.
The idea of using the size of 1 sensor pixel as explained by Michael Clark makes good sense to me, so I'm confused by the other website's idea above that it's 2 pixels, it's seems like everything else in that article is accurate and well said, it's hard to believe he's incorrect about the minimum CoC size.
j-g-faustus said and seems to imply diffraction/airy disk size is related, is he correct in saying 'the point where you can no longer tell two airy disks apart doesn't happen until the airy disk diameter reaches 2 pixels" -- I thought the airy disk became a problem when it was larger than 1 pixel or larger than the image's CoC???
@DavyCrockett I think using two pixels makes sense, by analogy with the diffraction limit - the point where you can no longer tell two airy disks apart doesn't happen until the airy disk diameter reaches two pixels. Similarly, a CoC of more than 1 pixel will bleed over and reduce the contrast between a pixel and its neighbour, but actual Confusion, the point where you can't tell two pixels apart, doesn't happen until CoC reaches two pixels. That would be my best guess, anyway