The Histogram Is Your Guide

You can use the bar chart (also known as a histogram) at the top of the Levels dialog box to determine whether the adjustments you're making are going to harm the image or improve it. The histogram indicates which shades of gray your image uses and how prevalent those shades are within the image. If you find a gap in the histogram, you can look at the gradient directly below it to see which shade of gray is missing from your image.

By looking directly below the first bar that appears on the left end of the histogram, you can determine the darkest shade of gray in the image. If there were anything darker than that, then there would have to be some bars above those shades in the histogram. By looking directly below ...

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