Levels

The information in the histogram is the best way to tell if an image is over or under-exposed. Ideally, with conventional images, you want the graph to extend neatly within the extreme boundaries of the histogram. If you find your histogram has minimal content at either limit, you can be safe to assume that the contrast of your image could be improved, and this is where Levels come into play."
Whatever we choose to do to the black point slider will have no effect on our highlights, and whatever change we make to the white point slider will leave the shadow end unaffected. Being able to adjust both ends independently gives us a great deal more control than does Brightness and Contrast.

hing which stretches the image histogram increases the possibility of posterization.
Performing levels on a luminance histogram can easily clip an individual color channel, although this may also allow for darker and brighter black and white points, respectively.
Performing levels on an individual color histogram or channel can adversely affect the color balance, so color channel levels should only be performed when necessary or intentional color shifts are desired.