Levels and Curves are the two fundamental Photoshop tools for global tonal and color correction. Levels is the easier of the two for beginning Photoshop users, and (in some situations) for experienced ones, too. Curves is a little more difficult to master, but a lot more powerful. We liken Levels to an automatic transmission and Curves to a stick shift. Levels is quick and easy. Curves lets you do all the same things (and more), but it demands a bit more skill, coordination, and experience.
Levels and Curves let you change existing (input) pixel values to new (output) pixel valuesbut they offer different ways of controlling the relationship between input and output. They share an important property that differentiates them from the Brightness and Contrast controls: They let you apply nonlinear transformations, instead of the linear transformations applied by the Brightness and Contrast controls (see the sidebar "The Nonlinear Advantage," later in this chapter).