tone reproduction
The Zone System and in general every rule that matches the dynamic range of the capture medium and the maximum contrast of the scene tries to get two results: correlating the minimum and maximum points of subject contrast with the dynamic range/contrast index of the recording material and to record the maximum number of tones between these extremes. When working with black-and-white materials (Leica Monochrom and black-and-white silver emulsions) a third parameter is required: the spectral sensitivity of the material must equal the spectral sensitivity of the eye. This introduces a subjective factor, because every human will have a different perception of the brightness of colors.
http://www.imx.nl/photo/technique/styled–10/colorimetryM9.html