You
            can set the gray values of an image by setting the gamma to brighten
            or darken an image without greatly affecting the shadows or highlights.
         
         To set the gamma of the image:
         
         
            - Select the tonal range that you want
               to modify.
            
- Drag the Gamma field to change the value
               (you can also type values into the field). Lower the gamma value
               to increase the contrast or raise the gamma value to decrease the
               contrast.