ABSTRACT
Focusing is the task of locating the image surface which best permits the maximum transfer of information in an optical image to another system – visual, optical, photographic or opto-electronic. Assuming that a lens has an accurately calibrated focusing scale, estimation of subject distance and use of depth of field tables or scales is a reasonable method. For low-cost cameras, a stadiametric rangefinder device, which may have no or few uncritical moving parts, is occasionally used. A major disadvantage of a coupled rangefmder is that it cannot easily be used in the close focusing range of a lens without special supplementary prisms or devices to give in effect the large optical movements needed. In a viewfinder device, two index marks are set coincident with, say, the top and bottom of the dimension used, the separation of the indices being calibrated as subject distance.
