Abridgment: Safety Factors in Camera Exposures

Abstract

A new American Standard for determining the speeds of black-and-white negative films for still cameras was issued in April 1960 by the American Standards Association. The new speeds are approximately twice the exposure indexes defined by the older version of the Standard. The level of the numbers was increased in order to reduce the “safety factor” in camera exposures indicated by exposure meters. The reduced exposures give negatives that are better from the standpoint of ease of printing, sharpness, and print graininess. The present paper is a report on a study, made in connection with this standardization project, to determine the absolute magnitude of the safety factor. Both the mathematical and experimental parts of this study indicate that the safety factor associated with the exposure index is approximately 2.4 for average sunlit scenes when accurate meters, shutters and lens apertures are used. The safety factor associated with the new ASA speed is slightly more than 1.2, the same as for color reversal films. The revised speed criterion and the new additive system of units for film speed, scene light, lens aperture and shutter speed are described.