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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. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「APEX system」の詳細全文を読む スポンサード リンク
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