APEX system

APEX stands for Additive system of Photographic EXposure, which was proposed in the 1960 ASA standard for monochrome film speed, ASA PH2.5-1960, as a means of simplifying exposure computation.

Exposure equation

Until the late 1960s, cameras did not have built-in exposure meters, and many photographers did not have external exposure meters. Consequently, it often was necessary to calculate exposure from lighting conditions. The relationship of recommended photographic exposure to a scene's average luminance is given by the camera exposure equation

where

is the relative aperture (f-number)

is the exposure time ("shutter speed") in seconds

is the average scene luminance ("brightness")

is the ASA arithmetic film speed

is the reflected-light meter calibration constant

Use of the symbol for luminance reflects photographic industry practice at the time of ASA PH2.5-1960; current SI practice prefers the symbol . German sources typically used for the relative aperture. Many authors now use and for relative aperture and exposure time.