I want to work out depth of field for various formats, focal lengths, and focussing distances. The formulas are straightforward enough, but they all depend on the diameter of the circle of confusion. According to Kodak's Large Format book, this diameter is about 1/1000 of the focal length of the normal lens for the format (and multiply the result by 0.6 for critical use). Now this seems rather crude to me, because it doesn't take into account how much the film will be enlarged in the print, or how far the viewer will stand from the print. Are there more sophisticated formulas that take these factors into account? If not, let me be more specific: I want the diameter of the circle of confusion for no enlargement (i.e., contact prints) viewed at the viewer's minimum focussing distance. My intuition says that it shouldn't depend on format, e.g, for 8 x 10 and 20 x 24, the figures should be the same. Does this make any sense?