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.
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.
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 Friedrich Deckel in the early 1950s.
, 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,
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
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 the other to maintain a constant exposure.
On some models, the coupling of shutter speed and aperture setting was optional, so that photographers could choose their preferred method of working depending on the situation.
This can be found in features such as Manual Shift on some Minolta, Konica Minolta and Sony Alpha or Hyper Manual on some Pentax (D)SLRs since 1991, where the photographer can change one of the parameters, and the camera will adjust the other accordingly for as long as the Auto-Exposure Lock (AEL) function is activated.
Although some photographers (Adams 1981, 66)[1] routinely determined camera settings using the exposure equation, it generally was assumed that doing so would prove too daunting for the casual photographer.
Taking base-2 logarithms of both sides of the exposure equation and separating numerators and denominators reduces exposure calculation to a matter of addition: where ASA standards covered incident-light meters as well as reflected-light meters; the incident-light exposure equation is where The use of
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: where (German sources typically use
However, the logarithmic markings for aperture and shutter speed required to set the computed exposure were never incorporated in consumer cameras.
Accordingly, no reference to APEX was made in ANSI PH3.49-1971 (though it was included in the Appendix).
With the passage of time, formatting of APEX quantities has varied considerably; although the
originally was subscript, it sometimes was given simply as lower case, and sometimes as uppercase.
for exposure, luminance, and illuminance) used at the time APEX was proposed are in conflict with current preferred SI practice.
Canon, Pentax and Leica 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 ISO 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.
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).
The sense of exposure compensation is opposite that of the EV scale itself.
There are some minor differences from the original APEX in both terminology and values.
: Because Exif 2.2 records ISO arithmetic speed rather than film sensitivity, the value of
in cd/m2, this becomes 10.7, which is slightly less than the value of 12.5 recommended by ISO 2720:1974 and currently used by many manufacturers.
in the example table; it also is possible that the example data simply were copied from an old ASA or ANSI standard.
To use the Exposure Formula, take the film speed number (on the ASA scale) and determine its approximate square root.
... At the key stop, the correct shutter speed is the reciprocal of the luminance expressed in c/ft2.Using these values for