Re-evaluation of Factors Affecting Manual or Automatic Control of Camera Exposure

The sophistication of exposure controls in cameras has demanded more thorough analysis of the predetermination of exposure. The film exposure level maintained by an automatic control in a camera depends primarily on the film speed but several other variables can manifest substantial influence. The effects of field-luminance distribution and spectral sensitivity, as well as the sensitometric, optical and photometric relations are expressed analytically and the equations for camera exposure are derived. The resulting constant relates the ASA standard film speed to the preferred exposure for an area in an average scene having the average luminance indicated by the meter. This constant, when combined with nine variables which are a function of camera design, meter design and scene structure, provides an equation that is simplified by substituting empirical values for all but three parameters. The exposure constant is expressed as a function of the lens transmission, spectral characteristics of the detector and the discrimination of the field luminance measurement.