The images in this gallery show the affect as seen in the histogram of using Levels, Curves, and other controls in image editors to redistibute tones in a photograph for improved dynamic range.

Basically, a histogram shows a graph that represents how the tones in an image are distributed, with pure black on the left, midtones in the middle, and pure white on the right.

The ‘Levels’ command provides adjustment points at each of those three positions and sliding those points along the baseline of the histogram will change where black, middle, and white tones are located in the range of tones of the original image.

When post-processing, I highly recommend using adjustment layers for such commands as Levels, because changes made with other commands can also affect how the tones are distributed, therefore affecting how the tones appear in the histogram, and can be easily corrected if Levels are being set with an adjustment layer.