Understanding the Histogram
Used properly, the histogram that's displayed behind the curve in Photoshop's Curves dialog can help you to make sure that your adjustments don't get out of control and end up harming your images instead of improving them. The Histogram panel (see Figure 1) also can help you to ensure that you don't push your edits and adjustments too far. To display the Histogram panel, choose Window > Histogram.
Figure 1 The Histogram panel.
A histogram is a simple bar graph that shows the range of brightness levels that make up an image and the prevalence of each of these shades. When you look at the histogram, imagine that a gradient is stretched across the bottom of the bar graph, with black on the left and white on the right (see Figure 2). If the histogram shows a bar above a particular shade of gray, that shade is used somewhere in the image. If there's no bar, that brightness level isn't used in that image. The height of the bar indicates how prevalent a particular brightness level is, compared to the others that make up the image.
Figure 2 Imagine a gradient at the bottom of the Histogram panel.
The Histogram panel includes a bonus. When you start to adjust an image, the Histogram panel overlays a histogram that represents the current, adjusted state of the image (black) above the original histogram that shows what the image looked like before you started adjusting it (gray), as shown in Figure 3.
Figure 3 The gray histogram reflects the unadjusted image; the black version reflects the adjusted image.