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Avoiding Posterization
If the histogram in the Histogram panel is showing gaps that make it look like a comb (see Figure 9), keep an eye on the brightness levels directly below that area of the histogram. Gaps in a histogram indicate that certain brightness levels are not found in the image, which can indicate posterization (stair-stepped transitions where there would usually be a smooth transition), as in Figure 10. That usually happens when you make part of a curve rather steep. As long as the gaps are small (two to three pixels wide), it's unlikely that you'll notice it in the image. If the gaps get much wider than that, you might want to inspect the image and think about making the curve less steep. The histogram in the Curves dialog doesn't show these gaps, because it only shows the original, unedited histogram.
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Figure 9 A histogram that looks like this might indicate that the image is posterized.
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Figure 10 The posterized image based on the histogram in Figure 9.
To better understand posterization, try this: Create a new grayscale document, press D to reset the foreground and background colors to black and white, and then click and drag across the document with the Gradient tool. While watching the Histogram panel, choose Image > Adjustments > Posterize and experiment with different settings—the gaps don't have to be very wide before you notice posterization (see Figure 11).
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Figure 11 Posterize a grayscale image to get a sense of how wide the gaps can be before you see posterization in an image.
You can minimize posterization by working with 16-bit images. Unlike standard 8-bit images that are made from 256 shades of gray (or 256 shades each of red, green, and blue), 16-bit images contain up to 32,767 shades of gray. You can obtain 16-bit images from RAW format digital camera files when opening them in the Camera Raw dialog, or from some newer flatbed or film scanners.
The Histogram panel usually builds its histogram by analyzing an 8-bit cached image, just to make sure that the panel display updates quickly. A cached image is a smaller version of the image with 8 bits of information. If you notice the "comb" look when adjusting a 16-bit image (see Figure 12), look for the warning triangle near the upper right of the histogram. That symbol indicates that the histogram is being created from a lower-resolution 8-bit image. Clicking the triangle causes the histogram to be redrawn directly from the high-resolution 16-bit file, which should eliminate the comb look and therefore indicate that the image isn't really posterized (see Figure 13).
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Figure 12 This histogram indicates that the image might be posterized.
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Figure 13 The uncached histogram is a more accurate view of the image.