SMPTE 243M-1993

This standard specifies the lens markings, focal length tolerances, mounting diameters, mechanical factors in mounting additional lens adapters to lenses, and the preferred value steps in focal lengths for lenses used in 35- and 70-mm motion-picture projectors . (Focal length referred to in this standard is the equivalent focal length, commonly known as EFL.)This standard also specifies the limiting or maximum available space for projector lens mounts where lenses of varying focal lengths and designs and attachments thereto are used.