Creative Image Editing in Photoshop - Black and White in Photoshop CS4 and Photoshop Lightroom - page 205

The Gamma slider, the middle slider, allows you to change the midtone
values within the image without af ecting the highlight or the shadow values.
Moving the Gamma slider maps where the middle gray, or 128 tonal value,
will fall. Click OK, and Photoshop remaps the pixel data to your specii cations.
Keep a close eye on the histogram as you make corrections in the digital
darkroom to make sure that your editing is not creating signii cant gaps in
the histogram. Too much pixel pushing can lead to posterization, which is the
lack of smooth graduation between tones. (See “The Zone System Applied:
Exposure Evaluation with Histograms“, page 41 for a more in-depth analysis of
the histogram)
Output Levels
Output Levels also af ect the brightness and contrast of the image. This
control slider compresses the tonal range of an image. Moving the highlight
output slider to the left decreases contrast and darkens the image. Moving
the shadows output slider to the right also decreases contrast, but lightens
the image. This slider is most often used for graphic design purposes in
screening back an image, to overlay text for example.
Preview
Turning on and of the preview checkbox allows you to view the changes you
made to the image before you apply them.