A figure of merit, called the Transfer Efficiency, may be calculated or measured for any camera based upon the exposure and the radiance of the subject required to produce a density of 1.0 on the film. The Transfer Efficiency of a camera serves to calibrate it so that the luminosity of events may be determined from measurements of film density. It also allows one to know in advance which experiments may be instrumented with a particular camera. Basic photometric units and measurements are discussed. The Transfer Efficiency is calculated for several representative cameras, including a Kerr cell camera, a rotating mirror camera, and the STL Image Converter Camera.