Requirement 0 – Our expectations from the contrast reproduction of a photograph

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What do we expect from a photograph? – If we leave all more or less oblique specifications aside, we can safely agree that a photograph should depict the subject realistically.

To meet the criterion of realism, the perceived contrast must be in a ratio of 1:1 to the subject contrast. Anything twice as bright in the subject must appear twice as bright in the image. At first glance, one might assume that the image must reproduce the tonal values with exactly the same luminances as the motif, resulting in a 100% correspondence. This requirement is very difficult to fulfill, because this would only be the case if we looked at the image under the same conditions as the subject. However, we usually look at our photos in the rather poorly lit parlor rather than on the sun-drenched rim of the Grand Canyon. Fortunately, this hurdle is not a problem, because in this respect it is exceptionally convenient that our visual system does not classify objects on the basis of their absolute brightnesses, but that the relative brightnesses suffice for it. Therefore, we only need to transfer the contrast, not the intensities, 1:1. This implies that the viewer must perceive what is twice as bright in the motif as being twice as bright. To put it another way, we must transport the contrast in a way that aligns the tonal values in the viewer’s perception with those in the motif. In this way, we can create the impression of the real scene because the visual system also constructs the brightnesses only from their ratios (see the chapter about brightness and color – Second Processing Stage: Conversion of Signals into Opposite Color Channels). Thus, if we consider our image under illumination conditions that differ from those of the original scene only in terms of absolute illumination intensity, it is sufficient to reproduce the tonal values with relative distance from the starting point white.

At the very beginning, we saw that an average motif has a contrast range of 400:1, but our output media can in many cases only reproduce a dynamic range of 275:1 because they do not achieve sufficient maximum density. This demonstrates that we cannot fulfill the initial requirement for the entire subject area. Instead, we have to decide on one area – shadows, midtones or highlights – and lower the contrast at the other two ends accordingly. As a rule, this will be the midtones, because, as we learned in the section „The minimum size of brightness differences“ our discrimination is most pronounced in the areas of medium brightness and gradually diminishes above and below that. Moreover, we will see that the requirement for compression in the shadows and highlights corresponds to the natural behavior of silver-based image carriers.

Next Factors we must take into account to meet requirement 0: Factor 1 – Stray light

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