Levels and Curves: Photoshop's Left and Right Ventricles Pump Luminance

Martini Hour 058, In Which Colleen Levels Out

Photoshop Top 40, Feature #9: Levels

The Levels command, and its cohort the histogram, let you adjust luminance levels on a channel-by-channel basis. The upshot is that you can increase contrast, correct for color cast, and make a bad image good.