APEX (photography)

See also: APEX

APEX (acronym of “ has off dditive system P hotography EX posure”) is a method of calculating of the duration based to a measure in Ev (Exposure been worth) adopted by the majority of the photographic exposure meters.

It uses a scale logarithmic curve easy to handle since calculations are limited to the addition.

Definitions

In this system, one with the following relation:

$Ev\; =\; AV\; +\; TV\; =\; Bv\; +\; Sv\; \backslash ,$

with:

• Ev: Exposure ( Exposure been worth )
• AV: Opening ( Aperture )
• TV: Exposure time or shutter speed ( Exposure time or Shutter Speed )
• Bv: Brightness ( Brightness )
• Sv: Sensitivity ( Film Speed )

AV is connected to the opening (out of F/) by the following relation: $Av\; =\; 2*\; \backslash \; log\_2\; (Opening)\; \backslash ,$

TV is connected to the duration (in second) by the following relation: $Tv\; =\; -\; \backslash \; log\_2\; (TempsExposition)\; \backslash ,$

Sv is connected to the sensitivity of film in ISO by the following relation: $Sv\; =\; \backslash \; log\_2\; \backslash \; left\; (\backslash \; frac\; \{ISO\}\; \{3,125\}\; \backslash \; right)$

Bv is connected to the brightness of the subject by the following relation: $Bv\; =\; \backslash \; log\_2\; \backslash \; left\; (\backslash \; frac\; \{B\}\; \{0,3*K\}\; \backslash \; right)$
where K=11,4 if brightness is expressed in candela/m {{2}} (cd/m) or K=3,33 if brightness is expressed in foot-lambert.

To calculate a log₂: $\backslash \; log\_2\; (X)\; =\; \backslash \; frac\; \{\backslash \; ln\; (X)\}\{\backslash \; ln\; (2)\}$ where ln is the Napierian logarithm, available on all the scientific calculators and the data-processing spreadsheves.

Use

Each increment of a unit means that the quantity of light is reduced by half. Each decrement of a unit doubles the quantity of light.

For example, to increase the opening of the optics of F/2 (Av=2) to F/2,8 (Av=3) reduced the light of half.

Another example: when one passes from a film of sensitivity 100 ISO (Sv=5) to a film of 400 ISO (Sv=7), the quantity of light necessary to obtain the same exposure is reduced to the quarter of the initial quantity.

Case of use

That is to say a brightness of the subject of 14,6 cd/m (Bv=2); with a film of 400 ISO (Sv=7), the value of Ev exposure is worth 9. To obtain a correct exposure:

• with a duration of 1/125 S (Tv=7), the opening must be F/2 (Av=2).
• with a duration of 1/250 S (Tv=8), the opening must be F/1,4 (Av=1).

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