APEX system (English Wikipedia)

Analysis of information sources in references of the Wikipedia article "APEX system" in English language version.

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iso.org

Ansel Adams described the exposure equation in a slightly different form:
To use the Exposure Formula, take the film speed number (on the ASA scale) and determine its approximate square root. This number is remembered as the key stop for that film. ... At the key stop, the correct shutter speed is the reciprocal of the luminance expressed in c/ft².

Using these values for $A$ and $T$ gives

${\frac {A^{2}}{T}}={\frac {\left({\sqrt {S}}\right)^{2}}{1/L}}=LS\,.$

Ostensibly, the calibration constant $K$ is missing, but with luminance in cd/ft², the value was unity (ASA Z38.2.6-1948 specified a range of 1–1.35; ASA PH2.12-1961 specified 1.06 ± 0.16. The current recommendation in ISO 2720:1974 would be 0.98–1.24 with luminance in cd/ft²).
ASA Z38.2.6-1948. American Standard for General-Purpose Photographic Exposure Meters (Photoelectric Type). New York: American Standards Association. Superseded by ASA PH2.12-1957. ASA PH2.12-1961. American Standard, General-Purpose Photographic Exposure Meters (photoelectric type). New York: American Standards Association. Superseded by ANSI PH3.49-1971. ISO 2720:1974. General Purpose Photographic Exposure Meters (Photoelectric Type)—Guide to Product Specification. International Organization for Standardization.