b. Optical Center. The optical center of a lens is a point, usually
(although not always) within a lens, at which the rays of light from two
different sources entering the lens are assumed to cross (fig 2-2).
Infinity is a distance so far removed from the camera
lens that the rays of light reflected to the lens from a point at that
distance may be regarded as parallel. Infinity is expressed by the symbol "
" and is a setting on a lens focusing scale.
2. Photographic lenses are measured according to their focal length which
is normally imprinted somewhere on the lens mounting, usually on the front
surface of the lens barrel (fig 2-3).
a. This focal length information is given in inches or in millimeters.
b. Focal length is frequently employed to indicate the size of a lens.