Using Live Histograms

As you learned from working with the Levels command, histograms are a useful tool for displaying information about the number of tones present in an image at each brightness level. A histogram consists of a series of up to 256 different vertical lines in a graph, arranged horizontally with the black tones represented on the left side of the graph, the white tones at the right side, and the middle tones located (you guessed it) in the middle of

Thisdocumentis createdwithtrial version of CHM2PDFPilot2.16.100.sent at that brightness level. A typical histogram has one or more peaks, and the black-and-white tones often don't extend to the theoretical limits possible for the image (0 for pure black at the left side and 255 for pure white at the right side).

Your digital camera may provide a histogram similar to the one shown in Photoshop, although they are used a little differently. With a camera, the histogram is used primarily to judge exposure and your main remedy is to increase or decrease exposure for the next picture you take under the same conditions. Within Photoshop, the histogram can actually be used to provide tonal corrections after the fact.

Figure 6.29 shows an image and its histogram. The tones are spread fairly evenly throughout the picture, and an eye with even a little experience can "read" this histogram fairly easily. For example, that little bump of tones at the far right of the histogram corresponds to the lightest tones of the image, the sky and the brightest leaves. The small number of tones at the far left correspond to the sparse distribution of very dark tones, chiefly in the shadows.

Figure 6.29. The curves of the histogram tell a story about the photo's tonal values.

Figure 6.29. The curves of the histogram tell a story about the photo's tonal values.

An overexposed photo might have most of the tones concentrated at the right side of the histogram, and an underexposed photo would probably have most tones concentrated at the left side. The Levels command, discussed earlier in this book, lets you make adjustments for these lopsided distributions, and includes its own histogram display.

Photoshop CS 1.0 had a brand-new Histogram Palette, which provides a lot more information to work with. You can see this palette's "live" histogram display in Figure 6.30. Several views are available; the illustration shows the expanded view with combined RGB histogram at the top, and separate red, green, and blue channel histograms arrayed at the bottom. In between are some information readouts that I'll explain shortly. You can also select an expanded view that hides the separate RGB histograms, and a basic version with no extra data, as shown in Figure 6.29.

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