The dynamic range of the visual system

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From the starless night sky with a luminous intensity of 4*10^-6 cd/m² (=0,000004 cd/m²) to the sun standing in the zenith, which has a luminous intensity of 3,2*10⁶ cd/m² (= 320,000,000 cd/m²), the enormous value of 12 log₁₀ units results in the way our visual system works. One log₁₀ unit covers about 3 exposure levels, so this is a good 36 exposure steps. The light levels are a bit abstract, aren’t they? To make it more understandable: For example, the full moon produces a luminance of 0.0001 cd/m² on a piece of paper. The limit for normal color vision is a luminance of about 0.01 cd/m². We can usually read comfortably from a luminance of 1 cd/m², and an uncovered daytime sky produces a luminance of about 1,000,000 cd/m². These values are pure madness when we think of our analog and digital recording materials. Reversal film can reproduce a modest contrast range of 1:64 or 6 exposure stops in projection, while negative material can handle a good 10 stops, and digital technology has now achieved a good 12 exposure stops. It is no wonder, then, that we are so often disappointed with our image results and have to hold back a sky that is too bright with a graduated gray filter in order to preserve the details of the foreground that are already in shadow. The visual system achieves this great degree by combining several different factors. The signaling of a certain amount of contrast by the photoreceptors is one of them. The ability of the visual system to adjust its sensitivity at different levels is another.

Films and electronic image carriers cannot easily reproduce the large differences between light and dark that our visual system is capable of mastering and thus often cause disappointment.

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