I've just registered to reply to this thread. In ALL the discussions above, no one has mentioned print size. Why is that important? Because DoF is ONLY relevant when you actually render the image so you can see it. Depending on how big you render the image and how closely you view it, the DoF will change.
Huh? But surely DoFmaster gives precise figures - yes it does, but it does based on assumptions of how big you are printing and how far away you are viewing it. The figures aren't there directly, but they are captured by the CoC value. If you look up how CoC is derived, it is obtained from a standard size print from a standard distance and the thing that matters is how much you are magnifying the sensor image to the final print size. Change the assumptions and the DoF changes.
Don't believe me? Take a look here: http://en.wikipedia.org/wiki/Circle_of_confusion
How is CoC defined:
CoC (mm) = viewing distance (cm) / desired final-image resolution (lp/mm) for a 25 cm viewing distance / enlargement / 25
Given an output print size, a desired viewing distance and a resolvability measure, the ONLY thing that dictates CoC is the enlargement
. Bigger sensor for a given photo -> less enlargement->larger CoC. That's what defines DoF.
Try this. Print an image from a FF camera at 12x8. Now crop it so that it is the same sensor image area as a crop camera and print the resulting image at 12x8. The depth of field will change because you have altered the zoom factor. You are seeing more of the image magnified. Same photo - different DoF because you are magnifying it differently.