Retouching Tools: Levels

Levels is a tool in Photoshop and other image editing programs which can move and stretch the brightness levels of an image histogram. It has the power to adjust brightness, contrast, and tonal range by specifying the location of complete black, complete white, and midtones in a histogram. Since every photo's histogram is unique, there is no single way to adjust the levels for all your photos. A proper understanding of how to adjust the levels of an image histogram will help you better represent tones in the final image.

You use the Levels adjustment to correct the tonal range and color balance of an image by adjusting intensity levels of image shadows, midtones, and highlights. The Levels histogram is a visual guide for adjusting the image key tones. You can save Levels settings as a preset, then apply them to other images

The levels tool can move and stretch brightness levels in a histogram using three main components: a black point, white point and midtone slider. The position of the black and white point sliders redefine the histogram's "Input Levels" so they are mapped to the "Output Levels" (default is black (0) or white (255), respectively), whereas the midtone slider redefines the location of middle gray (128). Each slider is shown below as they appear in Photoshop's levels tool, with added blue labels for clarity: