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Why 256 Grays?

Of course, your computer, digital camera, scanner, or other component that works with digitized images doesn’t have to divide up the entire grayscale into 256 different tones. Other values are, in fact, used to express grayscales. A scanner may indeed capture 1024 different gray tones when grabbing an image of a black-and-white photo, and then select the best 256 tones out of that range to represent the photo. Before you became involved with computers, those numbers might have seemed arbitrary to you, but by now you recognize them as values that are convenient for programmers. For example, 256 different tones are the most that can be represented by a single eight-bit byte (from 00000000 to 11111111 in binary equals 0 to 255 in our decimal notation). So, the real reason why we work with 256-tone grayscales is that this range makes things a heck of a lot simpler for programmers, like those who created Photoshop.

Are 256 tones enough? In truth, studies have shown that the human eye can differentiate only about 30 to 60 different gray levels. A highly detailed subject with no large gradated areas can sometimes be represented by as few as 16 gray levels, as you can see in Figure 4.2, which has precisely 16 different gray tones—no more, no less.


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