Take a look at the curves dialog here. There is a line running at 45°. This is the curve. When it's straight it's telling you that what goes in, is what comes out. The curve is not changing any values.<br /> <br /> The most basic concept in curves is that you want the steepest part of the curve to be in the area of interest. Contrast gets our attention. But in order to steepen the curve in one section, it has to be flattened in another. So all moves in curves come at a cost. You are drawing attention to one area at the expense of another.