Image key
Some scanners, especially low-end desktop models, tend to compress the tonal range of an image and increase the contrast or difference between light and dark areas. If the contrast of your original is too high or too low, detail could be lost when it is reproduced. Images that have few dark areas or shadow tones are called high key. In contrast, a low key image consists of large areas of shadow and darker midtones (the tones that fall in between).
Some scanners allow you to manually override automatic exposure controls in those cases in which the images have these characteristics.
Imaging software applications use histograms or graphic displays to show the distribution of pixels inside the different tonal ranges of an image.
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