Factors we must take into account to meet requirement 0: Factor 2 – Ambient brightness

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The section on the minimum size of brightness differences has shown that our ability to recognize brightness differences depends on the ambient brightness. If the ambient brightness is low, our ability to distinguish is also lowered compared to the average brightness. We can also say that our contrast sensitivity decreases when the ambient brightness decreases. This fact is significant for contrast reproduction. This is because we perceive images that are viewed at low brightness and are adjusted so that the contrast is transferred 1:1 (i.e. their characteristic curve rises at a 45° angle) as dull and washed out.

The Goldberg rule (Goldberg gamma) states that prints and slides must have different gamma values due to the different viewing conditions in order to convey contrast perceptually correctly.

The case in which this connection plays the greatest role is the viewing of slides (or also television pictures or cinema films) in dark or mutedly bright environments. We must increase the contrast to appear realistic in these cases. If we view the slide using a magnifier on a light desk and the surrounding room has a moderately downward brightness variation, we can adjust the contrast to a gamma value of 1.25. We can consider televisions and computer monitors to have similar viewing conditions, allowing us to refer to them as dimly bright environments, which a gamma value of 1.25 can correct for. If, on the other hand, we view the image in a dark environment, as is the case when projecting slides onto a screen, the gamma value must be 1.5 (we then refer to this as a dark environment). Only in an averagely bright environment, as we should make it for viewing prints, a gamma of 1.0 is correct. – So this is the reason why slide films have such a steep gradation and the associated low exposure range. 

Emanuel Goldberg (active as a scientist and managing director at Zeiss Ikon, among others) derived the contrast specifications for these different viewing conditions from physiological findings as early as the 1930s. They are known as the Goldberg rule or Goldberg gamma.

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