Editing Exposure value (section)

==EV and APEX==

{{Main|APEX system}}

The ''Additive system of Photographic EXposure'' ([[APEX system|APEX]]) proposed in the 1960 [[American Standards Association|ASA]] standard for monochrome film speed, [[#CITEREFASA PH2.5-1960|ASA PH2.5-1960]], extended the concept of exposure value to all quantities in the exposure equation by taking base-2 logarithms, reducing application of the equation to simple addition and subtraction. In terms of exposure value, the left-hand side of the exposure equation became

:<math>E_v = A_v + T_v \,,</math>

where <var>A<sub>v</sub></var> (aperture value) and <var>T<sub>v</sub></var> (time value) were defined as:

:<math>A_v = \log_2 A^2</math>

and

:<math>T_v = \log_2 (1/T) \,,</math>

with

* <var>A</var> the relative aperture (f-number)
* <var>T</var> the exposure time ("shutter speed") in seconds<ref name="no_units"/>

<var>A<sub>v</sub></var> and <var>T<sub>v</sub></var> represent the numbers of stops from {{f/|1}} and 1 second, respectively.

Use of APEX required logarithmic markings on aperture and shutter controls, however, and these never were incorporated in consumer cameras. With the inclusion of built-in exposure meters in most cameras shortly after APEX was proposed, the need to use the exposure equation was eliminated, and APEX saw little actual use.

Though it remains of little interest to the end user, APEX has seen a partial resurrection in the [[Exif]] standard, which calls for storing exposure data using APEX values. See [[APEX system#Use of APEX values in Exif|Use of APEX values in Exif]] for additional discussion.