APEX (photography)

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The APEX system (English Additive System of Photographic Exposure ; German: Additive system for photographic exposure ) represents the essential values in photographic exposure as small whole numbers, mostly in the range 0 to 10. The system is logarithmic and thus enables it to determine desired exposure values ​​through simple additions and subtractions.

Example: If the APEX value for the exposure time is increased by 1 (which means halving the exposure time), the APEX value for the f-number must be decreased by 1 in order to obtain the same exposure under otherwise identical conditions.

The method was presented within the framework of the standards ASA PH2.5-1960 , ASA PH2.12-1961 and ANSI PH3.49-1971 .

The APEX values

Aperture value Av

The aperture value Av (English aperture value ) is zero for the aperture ratio f / 1 and increases with each aperture stop by exactly 1, while the F-number in each stage with 1.4 (more precisely ) is multiplied.

F-number 1 1.4 2 2.8 4th 5.6 8th 11 16 22nd 32
Aperture value Av 0 1 2 3 4th 5 6th 7th 8th 9 10

Time value Tv

The value Tv (English time value ) is zero for an exposure time of 1 second and increases with each halving of order. 1 The table shows the rounded shutter speeds customary in photography, which are not exactly halved.

Exposure time (s) 1 1 ⁄ 2 1 ⁄ 4 1 ⁄ 8 1 ⁄ 15 1 ⁄ 30 1 ⁄ 60 1 ⁄ 125 1 ⁄ 250 1 ⁄ 500 1 ⁄ 1000
Time value Tv 0 1 2 3 4th 5 6th 7th 8th 9 10

Light value Ev

This exposure meter from 1952 is set to a light value of 9 (red scale) and indicates combinations of exposure time and f-number that result in this light value.

The sum of the aperture value and time value gives the light value Ev ( exposure value ):

Ev = Av + ​​Tv

The higher the light value, the lower the “amount of light” that reaches the image receiver during the exposure.

Sensitivity value Sv

The sensitivity value Sv (English speed value ) is zero for a film / sensor sensitivity of 3 ISO (exactly: 3.125) and increases by 1 with every doubling.

Sensitivity (ISO) 3 6th 12 25th 50 100 200 400 800 1600 3200
Sensitivity value Sv 0 1 2 3 4th 5 6th 7th 8th 9 10

Brightness value Bv

The brightness value Bv (English brightnessvalue ) is the luminance from the photographed area and follows the historic unit footlambert . It is zero for 1 footlambert and increases by 1 with every doubling.

Luminance (ft.la.) 1 2 4th 8th 16 32 64 128 256 512 1024
Brightness value Bv 0 1 2 3 4th 5 6th 7th 8th 9 10

Expansion of the scales

The APEX values ​​can be continued up or down (into the negative range) as desired, whereby a difference in the APEX value of 1 represents a doubling / halving (except for the f-number) of the physical value. A sensitivity of 12800 ISO corresponds to Sv = 12, an exposure time of 8 s corresponds to Tv = −3, an aperture of 0.7 corresponds to Av = −1.

application

The equation applies to correct exposure

Av + Tv = Bv + Sv

The mutual dependencies of the exposure parameters are reduced to the simplest addition and subtraction.

In order to selectively under or over exposure, the correction value Cv can be included in the equation:

Av + Tv + Cv = Bv + Sv

For Cv = 1 there is overexposure by 1 Ev, which corresponds to 1 f-stop.

meaning

Despite its simplicity, the APEX system has not been able to assert itself as far as the settings of photographic cameras - there the (rounded) physical values ​​for exposure time and f-number are usually used. On the other hand, the Munich precision mechanics manufacturer Friedrich Deckel designed camera shutters as early as the 1950s that had to be set according to a similar system called LVS (light value scale).

A lot of work is done with the light value Ev in professional exposure measurement. It also serves as a pseudo-unit for setting the exposure compensation in automatic cameras.

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