Depth of Field

Depth of field collapses as 1/m², so a close-up job has almost none. Stopping down buys depth linearly — until diffraction takes over and the Airy disc grows past a pixel, at which point a smaller aperture only makes things blurrier. Those two effects bracket the usable aperture.

Circle of confusion is usually taken as one to two pixel pitches. This is the close-up (macro) form of the DOF equation, which is the right one for machine vision; classical photographic DOF formulas assume distant subjects.