Nitty-Gritty on Program Exposure Settings
An exposure automation program used in many cameras, digital or not, uses an algorithm adjusting the shutter speed and aperture depending on scene luminance (brightness). Out of many possible such algorithms, one of the simplest and most predictable is the 45-degree scheme used, I believe, in most cameras.
The principle of this algorithm is simple. Starting from the lowest luminance (EV value) and progressing towards higher values, the exposure parameters are adjusted as follows:
To compute shutter speeds and apertures set by a 45-degree exposure automation program, you need five data items:
The first step will be to compute the three transition points of the program curve as EV values. Let us denote these as v1-2, v2-3, v3-4. The program will use aperture a1 for light values below v1, on the other extreme it will use a2 above v2, adjusting both shutter speed and aperture between v1 and v2.
The value of v1 can be computed as
v1 = log2 (s1a12)
where log2 stands for logarithm base two; if your calculator does not offer this option (mine does), you may compute log2x as log x/log 2, with log being either decimal or natural logarithm.
In a similar way, v2 will be
v1 = log2 (s1a24/a12)
For a given EV value, v, the program-selected aperture and shutter values are given in one of four ways:
where a is the F-number value and s — the shutter speed denominator (so that s=125 means
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