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As you've seen, Variations is the quick way to adjust color, but sometimes it doesn't give you enough control. Other times you just want to experiment. Maybe you have a picture that's mediocre, but if you play with the colors in it and beef up the contrast, you can make something out of it. These are the times when you'll want to work with individual adjustment settings.

 There's a menu item under the Image menu called Histogram. It doesn't actually do anything, but if you learn how to use it, you can save yourself lots of time. If you ever took a course in statistics, you already know that a Histogram is a kind of graph. In Photoshop, it's a graph of the image reduced to grayscale, with lines to indicate the number of pixels at each step in the gray scale from 0 to 255. You might wonder why this is important. The main reason is that you can tell by looking at the Histogram whether there's enough detail in the image, so that you can apply corrections successfully. If you have an apparently bad photo or a bad scan, studying the Histogram will tell you whether it's worth working on or whether you should throw away the image and start over. If all the lines are bunched up at one end of the graph, you probably can't save the picture by adjusting it. If, on the other hand, you have a reasonably well spread out Histogram, there's a wide enough range of values to suggest that the picture can be saved. The Histogram has another use, which is to give you a sense of the tonal range of the image. This is sometimes referred to as the key type. An image is said to be either low key, average key, or high key, depending on whether it has a preponderance of dark, middle, or light tones. A picture that is all middle gray would have only one line in its Histogram, and it would fall right in the middle. All you really need to know is that, when you look at the Histogram, you should see a fairly even distribution across the graph, if the image is intended to be an average key picture. If the picture is high key, most of the lines in the Histogram are concentrated on the right side with a few on the left. If it is low key, most of the values will be to the left with a few to the right.

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