1. An optical system defining an optical path that comprises: (a) a source of electromagnetic energy in the optical path; (b) an optical element in the optical path; (c) at least one of an object mask that is located near the object plane of the optical system or an image mask that is located near the image plane of the optical system wherein the at least one object mask or image mask comprises a patterned screen that defines an array of different sized apertures and a corresponding set of symbols that identify the different sized apertures in said array; and (d) an intensity detector.