Levels

The Levels tool allows you to adjust the lighting in an image by adjusting the the black point, white point and midtones. It has two automatic adjustments: Auto Levels and Auto Levels and Colors (these are available only when working in the RGB color space).

The Levels adjustment panel.

Sliders Use the left and right sliders to adjust the black point and white point. This can increase the tonal range. The middle slider adjusts the midtones, which can make the image appear brighter or darker.

Channel Selector When working in RGB color space, the red, green and blue channels can be adjusted independently. Choose the channel you want to adjust and use the sliders.

Auto Processing The Auto Levels button will set the black and white points automatically. Auto Levels and Colors will adjust the RGB channels and set the black and white point to give an even white balance.

Show Clipping When the Show Clipping button is down, areas that are pure black will be highlighted in blue. Areas that are pure white will be highlighted in red.

Settings This will open another window. You can set the clipping points for the Auto Levels and Colors function, and you can adjust how the histogram is displayed.

Levels and Histogram Options dialog.