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Photoshop® CS4 Bible by Dan Moughamian, Simon Abrams, Stacy Cates

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9.4. Summary

This chapter described the purposes for and how to use histograms, the Levels command, and the Curves command.

Histograms are bar graphs that describe the tonal range in an image. They can be helpful in deciding what image adjustments are needed. Histograms show vertical bars of varying heights that represent the amounts of each brightness level in an image, measured from 0 (black) to 255 (white).

The Levels command and the Curves command can be used to make an image look lighter, darker, or more detailed. They can also be used for full-color composite or individual channel adjustments. Levels displays a histogram and offers 3 points of adjustment: shadow, gamma (midtones), and highlight. Curves also displays a histogram, but can have up to 14 points of adjustment. Levels uses the 0 (black) to 255 (white) scale. Curves uses either the 0 (black) to 255 (white) scale or the 0 percent to 100 percent ink level scale. Curves offers convenient ways to determine which parts of the curve correspond to specific parts of the image.

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