depth of focus

The depth of focus Eq. 14 may be regarded as the depth of field at the film side of the lens. It has a few properties in common with the depth of field, such as that it increases with the F-number, but there are also differences. Provided that the image magnification and F-number are kept the same, there is no dependencd on the focal length whatsoever. The dependence on the image magnification is weak, a humble factor of two between infinity focus (M=0) and lifesize portrayal (M=1). The significance of the depth of focus is found in relation to the precision of camera alignment and film flatness. On the assumption of accurate focussing and a properly aligned camera, the depth of focus indicates the tolerance of the photographic system with respect to film displacement. In other words, if the film departs from its proper position by a distance which brings it out of the depth of focus the subject is imaged with a COC larger than the permissible value and considered out of focus. Overall the depth of focus is smaller for miniature (digital) cameras than it is for larger formats, which puts more stringent demands on manufacturing tolerances.