I've noticed that several online and/or downloadable Depth of Field Calculators use different values for the acceptable CoC (Circle of Confusion) for some of the same camera models/sensor sizes. This, of course, leads to different DoF (Depth of Field) results when any particular lens and aperture are entered. Below the rest of this question is a quote from Russell McMahon's answer to this question: Analysis of a macro shot with very fast shutter speed
Since the acceptable CoC entered in the formulas affects the DoF calculated, what criteria is used to determine acceptable CoC? The two numbers (0.029mm for Full Frame and 0.018mm for APS-C) listed in the Wikipedia article for CoC seem to be based on a set viewing distance and print size. Several of the online DoF calculators use different CoC values for cameras with sensors the same size as the FF and APS-C used in the Wikipedia CoC article. Wouldn't the acceptable CoC change for prints of different sizes from the same size image or a web displayed image at varying screen sizes/resolutions? How would you calculate an acceptable CoC for a given veiwing size and distance?
f = focal length
N = Aperture f number
c = circle of confusion
s = subject distance (assumed >> f)
from Wikipedia DOF
c ~= 0.025mm for FF 35mm.
c ~= 0.018mm typical crop APSC.
see Wikipedia COC
Some more DOF calculators
Many provide hyperfocal distance as well.