Imatest - Using Uniformity Part 1

Deriving f-stop falloff

The f-stop falloff in the second plot is derived from the equations,
Pixel level = k1 luminanceγ ; Luminance = k2 pixel level1/γ and
F-stop loss = log2(luminance ratio) = 3.322 log10(luminance ratio)
where luminance ratio is the ratio of the maximum luminance to the luminance in the area of interest, for example, the mean value of the corners.

Example: The first and second figures, above, are derived from the same image file. In the first figure, the mean pixel level at the corners relative to the center is 0.666/0.905 = 0.736 (73.6%). Since γ is assumed to be 0.5 (fairly typical of encoding gamma of digital cameras, the exposure at the corners relative to the center is 0.7361/γ = 0 .7362 = 0.5416. The corresponding f-stop loss = log2(0.5416) = 3.322 log10(0.5416) = -0.885 f-stops. There is a slight discrepancy with the second figure, which calculates the mean at the corners (0.894 f-stops) after taking the logarithm to convert results into f-stops.
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