Achieving Optimal Contrast
If the histogram doesn't extend all the way from black to white, the image has a limited brightness range (see Figure 4 and Figure 5). When that's the case, you can usually move the upper-right and lower-left points on a curve toward the middle, which will widen the histogram (see Figure 6). As you do, keep an eye on the histogram. Most images will look their best when the histogram extends all the way across the area available, without producing any tall spikes on either end.
Figure 4 An image with limited brightness. (©2008 Dan Ablan.)
Figure 5 The histogram for the image in Figure 4.
Figure 6 The result of applying the curve to the image.
Two controls make this edit a little simpler. Notice that below the grayscale ramp beneath the curve are sliders for black point and white point. Moving these sliders is the same as adjusting the points on the end of the curve.