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Chapter 3. Changing and Adjusting Colors > Understanding histograms

Understanding histograms

The histogram is a graphic representation of the tonal range of an image. The lengths of the bars represent the number of pixels at each brightness level: from the darkest, on the left, to the lightest, on the right. If the bars on both sides extend all the way to the left and right edges of the histogram box, then the darkest pixels in the image are black, the lightest pixels are white, and the image is said to have a full tonal range (Figure 3.24). If, as in many images, the bars stop short of the edges, then the darkest and lightest pixels are some shade of gray, and the image may lack contrast (Figure 3.25). In extreme circumstances, the bars may be weighted heavily to the left or right, with the tonal range favoring either the shadows or highlights. Whatever the tonal range, the brightness and contrast of an image can be adjusted using sliders located beneath the histogram in the Levels dialog box (see “Adjusting Levels Manually” later in this chapter).

Figure 3.24. A photo displaying full tonal range, and its accompanying histogram. Note how the histogram extends all the way to the left and right, indicating that pure blacks are present in the darkest shadow areas and pure whites are present in the lightest highlight areas. The fairly uniform peaks and valleys throughout the middle portion of the histogram also indicates that there is sufficient pixel data present in the midtones.



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