jmcfadden
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This was asked in the Nikon forum this week as well, here is a neat explantion from Molson
The “Circle of Confusion” is an arbitrary factor, used as an aid in selecting an appropriate depth of field to provide subjectively “acceptable” sharpness for a desired enlargement size. It is a term that has been adopted as an attempt to describe the amount of subjective “unsharpness” that is acceptable to the human eye in a photographic print. It has ABSOLUTELY NO BEARING WHATSOEVER on the physical depth of field properties of a lens or camera format. As someone rightly pointed out, you can't change physics, even if you pretend you can by posting contrary notions on your web site.
What the authors of the popular Depth of Field calculator web site haven’t realized yet, is that with digital capture, the degree of enlargement required to produce a given size image is NOT a function of sensor size, as it was with film. The degree of “enlargement” required to make a specific size of print from a digital capture is dependant on the NUMBER OF PIXELS in the sensor used to capture the image (hint: that’s how you can make bigger prints with your 6 megapixel camera than you could with your old 3 megapixel camera). The size of each pixel is irrelevant, just as the size of film grain is irrelevant in depth-of-field and circle-of-confusion calculations.
Let me illustrate this with a real-world example. Slightly LESS "enlargement" is required to make an 8x12 print from a Nikon D2X file than from a Canon EOS 1Ds file, because the Nikon image contains more pixels (4288 x 2848 pixels, versus the Canon’s 4064 x 2704 pixels). The less the original image needs to be enlarged to produce the desired final print size, the larger the acceptable Circle of Confusion can be before it degrades the subjective appearance of the print. Therefore, since the Nikon D2X images require some 5% less enlargement than EOS 1Ds files to produce an identical print size, the acceptable Circle of Confusion for the D2X is then 5% LARGER than it is for the EOS 1Ds. Using the logic presented by the “full frame fanatics”, this would mean that the D2X has LESS depth of field than the full-frame EOS 1Ds, even though the 1Ds has a larger imaging sensor.
I won't start to delve into why interpolation is not the same as traditional analog enlargement, which makes the old Circle of Confusion arguments even weaker...
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. Actually, you should really consider the merits of non-existent cameras, lens, and printers to get to the bottom of this whole depth of field thing.