APEX system

 ( Photographic Techniques ) 

Using these values for $A$ and $T$ gives $$\backslash frac\; \{A^2\}\; \{T\}\; =\; \backslash frac\; \{\; \backslash left\; (\; \backslash sqrt\; S\; \backslash right\; )\; ^2\; \}\; \{1/L\}\; =\; LS\; \backslash ,.$$ Ostensibly, the calibration constant $K$ is missing, but with luminance in candela per square foot (, equivalent to (fL), or approximately 10.76 candelas per square meter ()), the value was unity (ASA Z38.2.6-1948 specified a range of ; ASA PH2.12-1961 specified . The current recommendation in would be with luminance in ). routinely determined camera settings using the exposure equation, it generally was assumed that doing so would prove too daunting for the casual photographer. The 1942 ASA exposure guide, ASA Z38.2.2-1942, featured a dial calculator, and revisions in 1949 and 1955 used a similar approach.

An alternative simplification also was possible: ASA PH2.5-1960 proposed extending the concept of exposure value to all exposure parameters. Taking base-2 logarithms of both sides of the exposure equation and separating numerators and denominators reduces exposure calculation to a matter of addition: $$E\_v\; =\; A\_v\; +\; T\_v\; =\; B\_v\; +\; S\_v\; \backslash ,,$$ where

• $A\_v$ is the aperture value:
• $T\_v$ is the time value:
• $E\_v$ is the exposure value:
• $S\_v$ is the speed value (aka sensitivity value), such that
• $B\_v$ is the luminance value (aka brightness value), such that ;
• $N$ is a constant that establishes the relationship between the ASA arithmetic film speed $S\_x$ and the ASA speed value with a value of (precisely, The origin of the value of $2^\{-7/4\}$ for $N$ is arcane, apparently so much so that ASA PH2.12-1961 included an explanation of what ASA PH2.5-1960 had intended. and
• $K$ is the reflected-light meter calibration constant.

ASA standards covered incident-light meters as well as reflected-light meters; the incident-light exposure equation is $$\backslash frac\; \{A^2\}\; \{T\}\; =\; \backslash frac\; \{I\; S\_x\}\; \{C\}\; \backslash ,,$$ where

• $I$ is the scene illuminance; and
• $C$ is the incident-light meter calibration constant.

The use of $I$ for illuminance reflects photographic industry practice at the time of the 1961 ASA standard for exposure meters, ASA PH2.12-1961; current SI practice prefers the symbol

ASA PH2.12-1961 included incident-light metering in the APEX concept: $$E\_v\; =\; A\_v\; +\; T\_v\; =\; I\_v\; +\; S\_v\; \backslash ,,$$ where $I\_v$ is the incident-light value:

(German sources typically use $LW$ (for Lichtwert or Belichtungswert but not to be confused with the English term light value) instead of the exposure value's symbol Consequently, the aperture value $A\_v$ is referred to as Blendenleitwert and the time value $T\_v$ as Zeitleitwert The film speed value $S\_v$ is named Empfindlichkeitsleitwert, and the brightness value $B\_v$ is known as Objekthelligkeit.)

With the passage of time, formatting of APEX quantities has varied considerably; although the $v$ originally was subscript, it sometimes was given simply as lower case, and sometimes as uppercase. Treating these quantities as rather than quantity symbols probably is reasonable because several of the quantity symbols and $I$ for exposure, luminance, and illuminance) used at the time APEX was proposed are in conflict with current preferred SI practice.

A few artifacts of APEX remain. Canon, Pentax and Leica Camera cameras use 'Av' and 'Tv' to indicate relative aperture and shutter speed as well as to symbolize aperture priority and shutter priority modes. Some Pentax DSLRs even provide a 'TAv' exposure mode to automatically set the Film speed depending on the desired aperture and shutter settings, and 'Sv' (for sensitivity priority) to pre-set the ISO speed and let the camera choose the other parameters. Some meters, such as Pentax , directly indicate the exposure value for ISO 100 film speed. For a given film speed, exposure value is directly related to luminance, although the relationship depends on the reflected-light meter calibration constant Most photographic equipment manufacturers specify metering sensitivities in EV at ISO 100 speed (the uppercase 'V' is almost universal).

It is common to express exposure increments in EV, as when adjusting exposure relative to what a light meter indicates (Ray 2000, 316). For example, an exposure compensation of +1 EV (or +1 step) means to increase exposure, by using either a longer exposure time or a smaller f-number. The sense of exposure compensation is opposite that of the EV scale itself. An in exposure corresponds to a in EV, so an exposure compensation of +1 EV results in a smaller EV; conversely, an exposure compensation of −1 EV results in a greater EV.

The relationship between $B\_\{\backslash mathrm\; \{v\}\}$ and luminance depends on both the speed scaling constant $N$ and the reflected-light meter calibration constant $$B\_\backslash mathrm\; \{v\}\; =\; \backslash log\_2\; \backslash frac\; \{B\}\; \{NK\}\; \backslash ,.$$

Because Exif 2.2 records ISO arithmetic speed rather than film sensitivity, the value of $N$ affects the recorded value of $B\_\{\backslash mathrm\; \{v\}\}$ but not the recorded film speed.

Exif 2.2 does not recommend a range of values for presumably leaving the choice to the equipment manufacturer. The example data in Annex C of Exif 2.2 give 1 footlambert for = 0}}. This is in agreement with the APEX value for but would imply or 3.125 with $B$ in footlamberts. With $B$ in cd/m, this becomes 10.7, which is slightly less than the value of 12.5 recommended by and currently used by many manufacturers. The difference possibly arises from rounding $B$ in the example table; it also is possible that the example data simply were copied from an old ASA or ANSI standard.