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 f-number (reciprocal of the relative aperture) 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. Recommendations for the value of the calibration constant in applicable ANSI and ISO standards have varied slightly over the years; this topic is discussed in greater detail under Exposure meter calibration in the Light meter article. Exposure value In an attempt to simplify choosing among combinations of equivalent camera settings, the concept of exposure values (German: Lichtwert) was originally developed and proposed to other manufacturers by the German shutter manufacturer in the early 1950s. Combinations of shutter speed and relative aperture that resulted in the same exposure were said to have the same exposure value , a base-2 logarithmic scale defined by When applied to the left-hand side of the exposure equation, denoted combinations of camera settings; when applied to the right-hand side, denoted combinations of luminance and film speed. For a given film speed, the recommended exposure value was determined solely by the luminance. Once the exposure value was determined, it could be directly set on cameras with an scale. Adjustment of exposure was simple, because a change of 1 corresponded to a change of 1 exposure step, i.e., either a halving or doubling of exposure. Starting 1954, the so-called Exposure Value Scale (EVS), originally known as Light Value Scale (LVS), was adopted by Rollei, Hasselblad, Voigtländer, Braun, Kodak, Seikosha, Aires, Konica, Olympus, Ricoh and others, introducing lenses with coupled shutters and apertures, such that, after setting the exposure value, adjusting either the shutter speed or aperture made a corresponding adjustment in