Originally posted by sundr Speaking of which...what is "circle of confusion?"
Google and the Wikipedia are our friends. I am not sure that Michael used the term appropriately in his comments, though he did use a VERY important word. That word is the term "acceptable". The circle of confusion is the diameter of the intersection of the out-of-focus optical cone with the focal plane. Maximum diameter of CoC is that number that will still yield an acceptably sharp image to the person viewing the photo at a reasonable viewing distance for a given final image size. It is that maximum that is used for calculation of DOF. Acceptable maximum CoC varies by the frame size of the capture media (say APS-C vs. 35mm FF vs. 4x5 film and so on) assuming that final image size and viewing distance are held constant. Oh, and one more thing, average visual acuity of the person viewing is assumed.
I can not emphasize too strongly that DOF is a measure of adequacy and is not absolute. In addition, it is relative to size and viewing distance. Stand too close or pixel peep and DOF disappears.
Steve