Diffraction and minimum aperture

Going back to aperture, simplistically it would seem that the narrower the aperture we use the larger the resulting DOF, and therefore an aperture as small as a pin hole would give us nearly unlimited DOF. Unfortunately, photons arriving at the very edge of the diaphragm opening tend to change paths and scatter, which is called diffraction:

For any focal length and subject distance, there is an aperture size, smaller than which diffraction robs any gains made by the shallower angle of incidence. And, of course, another consequence of reducing aperture is lengthening exposure time (to collect enough photons to expose the film or sensor). And the longer the exposure the greater the blur resulting from any subject motion.

Pragmatically, the smallest aperture before diffraction loss takes over varies with the film/sensor size. Experimentation has shown me that f/11 is the lower limit for 35mm; f/8 is commonly thought to be the lower limit for APS dSLRs, 4x5 view camera users routinely stop down to f/32; and at the opposite extreme, digicams don't even offer apertures smaller than about f/8.

Further reading

The above is a very simple introduction to the fundamentals of DOF. To answer your further questions I commend you to the following:

Cambridge in Colour: Understanding Depth of Field by Sean McHugh

Depth of Field Hell by Mike Johnston

Depth of Field Hell – The Sequel by Ctein

Depth of Field by Dr. Ching-Kuang Shene

DOF Master – Depth of Field Calculators by Don Fleming

Depth of Field and the Digital Domain by Bob Atkins.