Depth of focus is sometimes confused with depth of field, but the two terms are now generally used to describe quite separate distances. It is perhaps most easily understood when described as the extent to which the distance between a camera's lens and image plane may be varied whilst still maintaining an acceptably sharp image. In other words, it is the displacement of a film or sensor within a camera. For instance, if a frame of film was slightly curled as it extended across the image plane, some part of the image might be unsharp. Depth of focus defines the extent to which such an error might be tolerated without loss of acceptable sharpness. Acceptable sharpness is defined by a circle of blur (similar in nature to the circle of confusion used in defining depth of field).
Depth of focus decreases with wider lens apertures, but decreases with a shorter focal length or a more distant subject. The depth of focus for a standard lens is given by:
DF = (FL x A) / 1000
(where DF is the depth of focus, FL is the focal length of the lens, and A is the f/stop in use.)
The diagram below illustrates the measurement of depth of focus.