The number of tone values in a print

You are here: Nature Science Photography – Contrast – Contrast perception

For all the photographers reading this, I will address the question of how many tonal values can be perceived in a photograph. This value depends, on the one hand, on the dynamic range of the print and, on the other hand, on its illumination. A very good print reaches a maximum density of 2.0, which corresponds to a contrast of 100:1 (10² = 100) and a dynamic range of 2.0/0.3 = 6.6 exposure stops. In the best case, if our discrimination ability remained constant at 1% over the entire range, we could distinguish log 200/log 1.01 = 532 tonal values. However, our discrimination ability decreases by a factor of 8. We can therefore perceive 70 tonal values in the brightest exposure level (corresponds to 1/70 of an exposure level, because 1.01⁷⁰ = 2.0) and a good 9 in the darkest (70/8=8.75). Figure 22 shows a coordinate system in which the 70 tonal values are plotted on the y-axis and the exposure levels rounded to a full seven on the x-axis. A straight line connects the end points, on which we enter the number of perceptible tonal values per exposure level. Their addition results in a sum of 279.

We can assume the figure of a good 280 tonal values if we look at the print under the illuminance of 200-300 cd/m², which is considered optimal. For comparison: a sunny day brings up to 7000 cd/m², in offices there is usually 100 cd/m² and in the living room in the evening we measure between 20 and 40 cd/m². If we look at the same print once under 200 cd/m² and once under 20 cd/m², in the second case it will appear too dark overall, and we will only be able to perceive a good 75% of its tonal values, losing the most in the shadows. On the other hand, when we view a photograph in bright direct sunlight, we perceive it as too bright. Under these conditions, we will be able to see a lot of details in the dark areas of the image, but we will not be able to distinguish a lot of tonal values in the highlights. Everyone can see this for himself with a grayscale wedge under different ambient brightnesses. If we output the image slightly darker than normal and view it under illumination slightly brighter than average, we can positively utilize this connection. In this scenario, we can perceive a greater range of tonal values.

By the way, you can approximate the illumination level as follows using your camera’s light meter: Simply aim at a white sheet of paper under the same lighting conditions under which you are viewing the image. At a sensitivity setting of 160 ISO and an aperture of 5.6, the illuminance in cd/m² is pretty much the reciprocal of the exposure time. So if you read 1/500 sec, the illuminance is around 500 cd/m².

To prevent premature fading due to the high proportion of ultraviolet radiation, it is important to avoid permanently exposing a photograph to an illuminance of 300 cd/m². Framing under UV protective glass, such as Tru-Vue, provides reliable protection against this, filtering out 97% of UV light.

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