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Chapter 2. Tonal Correction > Determine tonal quality by reading a histogram

Determine tonal quality by reading a histogram

We've all had it happen—someone gives you a less-than-stellar image to use for a project you're working on. When this occurs, you should immediately evaluate the image's tonal quality. After all, you probably don't want to spend countless hours trying to correct the image in Photoshop only to find out later that it isn't salvageable. One of the ways you can determine if an image is of quality, or if it can be saved, is to view its histogram.

What's a histogram?

In Figure A, we show an image with its histogram. This is an example of a quality image that has a well-dispersed tonal range, meaning that it has an acceptable amount of pixel data in all three ranges—shadows, midtones, and highlights. A histogram is a graph, which represents the total tonal range of an image. The horizontal axis on a histogram shows the tonal range and the vertical axis shows the number of pixels, from 0 to 255, at each tonal value. The left side on the histogram shows shadow detail in an image; the right side shows its highlights; and all the detail in between are midtones. An image that's too dark, or underexposed, shows more detail on the left, and an image that's too light, or overexposed, shows more detail on the right.


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