Q Choose Imageo Adjustmentso Levels to view the changed histogram.

Notice that now all the image data tucks in just between the highlight and shadow markers. You can also see missing data points — the vertical white lines — caused by the redistribution of image data. With the data spread out over a wider range of gray, there aren't enough original grayscale values to fill all data slots.

Photoshop Confidential

^fty Adventures with ^Kissing Histogram Data

Every time you venture into (and make an adjustment with) an image-altering dialog box such as Levels, your histogram takes a hit in terms of image quality. For example, If you lose too much image data — and too many of those white spaces appear in the Levels histogram — your image may become posterized, resulting in a blotchy look to your image.

If you perform multiple rounds of image adjustment, your histogram data, and therefore your image, will show a progressively increasing loss of image data (represented by an increasing number of missing histogram data points). Moral: As a rule, the best policy is to perform as few edits as possible; do your best to get it right the first time!

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