Levels is a fundamental adjustment, present in all versions of Photoshop. It is used to quickly adjust the contrast and tone of an image.
The inclusion of a Levels-like tool usually sets an image editor in a class above others. Compared to Brightness/Contrast—often the only adjustment available in lesser image editors—Levels offers more precise control over the resulting dynamic range, as well as a way to independently alter the tone of the image.
Though not as realistic or precise as an Exposure adjustment, Levels is a quick way to touch up dim or overblown photos. Like the Curves adjustment, the total number of gray levels may decrease, causing banding at the extreme.
- Input black
- A black triangle indicating which gray level will be mapped to Output black; all values below this will be clipped.
- Input midtone
- Also known as gamma, a gray triangle indicating the gray level that will be mapped halfway between Output black and Output white; all other input values will be mapped according to a simple spline curve.
- Input white
- A white triangle indicating which gray level will be mapped to Output white; all values above this will be clipped.
The input sliders can expand the contrast in an image, while the gamma slider can adjust tone.
- Output black
- A black triangle indicating the darkest resulting gray value (up to black).
- Output white
- A white triangle indicating the lightest resulting gray value (up to white).
The two output sliders serve to constrain the resulting contrast, even permitting all colors to be mapped to a single gray value if so desired.
For most purposes, these sliders can be left alone, unless a particular range of grays should be excluded.