Levels

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Levels is a control in Adobe Photoshop and other software applications for correcting the exposure of an image using sliders on the histogram.

The right hand white control under the graph sets the highest occurring value, the left hand black control sets the lowest occurring value, and the middle control sets the midpoint. In normal use, the right control should be moved until it is directly underneath the highest occurring value, and the left control under the lowest occurring value. The centre control changes the gamma of the image.

The output levels can be set to reduce the dynamic range of the image, or, more usefully, to preview how the image will appear on an output device with low dynamic range, such as a mono laser printer or a Risograph.

The Eyedropper tools can be used to set the black point, white point and a neutral grey point, which changes the colour balance.

Typically, levels is used to control all three RGB values together, but each channel can be adjusted separately, although this will produce a colour shift in the image.