How to work with Levels
To work with Levels
Select the Choose Color tab from the Menu Bar and choose Adjust in the Menu. After that click Levels.
Modify the Input levels to darken the darkest values and lighten the lightest values. This increases the contrast in the image. The Input levels boxes correspond to the diamonds on the slider underneath them. The box and diamond on the left represent a lightness value of 0. The middle box and diamond control the gamma curve. The box and diamond on the right represent a lightness value of 255.
The Output levels modify the edited Input values to another set of brightness values. Modify the levels to lighten the darkest pixels and darken the lightest pixels. You set the lowest range and highest range for brightness levels.