This is a Very Deep look at Digital Camera Depth of Field

Depth of field for digital cameras: "Digital camera sensors (at least in the cameras below $1000 or so) are much smaller. For example, the active sensor area in the Olympus C-30x0Z series is about 5.27x7.03 mm.
(I arrived to these numbers based on the '35 mm equivalent' 32 mm focal length at the Oly 6.5 mm setting. Actually, if Olympus does not follow the customary rule of using the diagonal measure to define lens equivalence, the size may be a few percent different.)
A frame from such a camera printed at 9x12 inches (23x30 cm; note the somewhat different aspect ratio) requires a 44x magnification. Therefore the acceptable size of the circle of confusion will be almost exactly five times smaller, close to 0.006 mm. "