In the currently hidden thread (Image quality: Normal holders vs. Quickload), someone claimed, "For close subjects such as portraits, the lense extension will increase depth of focus sufficiently to negate any slight mis alignment of the film plane."
This has some truth in it, but the question is how much: How much more depth of focus do you get when you focus close? Using Merklinger's figure 5 from his book Ins and Outs of Focus (which can be downloaded as a PDF from the web), it looks like there is a simple answer. The effective aperture is all you need to know. For example, when focusing for 1:1 macro with an 80mm lens at f/16, your extension is an extra 80mm and your effective aperture is f/32. Your depth of focus in this case is equal to what it would have been had you been using f/32 and focusing at infinity.
It appears to me that depth of focus is determined by your circle of confusion and your effective aperture, and nothing else. Your focal length does not matter, and how close you focus does not matter (except insomuch as it affects your effective aperture).
I am not sure whether this result is exact or an approximation, so any comments or analysis on this would be appreciated. If this result is only an approximation, I would appreciate hearing how far off it can be, and why.