Critical focus

This does not relate to depth of field which describes apparent sharpness.

Reducing the size of the aperture will increase the depth of field but the plane of critical focus will not change.

The formula that describes the relationship between plane of sharpness, lens and film is where

is the distance from the lens to the plane of critical focus (in the object space), and

'Critical Focus' is also the title of a regular column by Brian J. Ford in the American magazine The Microscope.