Photo Levels - Using Photoshop.

The levels control shows a representation, in the form of a histogram, of the distribution of tone values of an image. Levels offers several powerful options for changing global tone distribution. The easiest thing to do is to click on the auto level button. This, however, works effectively in very few cases. What it does is take the darkest pixel to maximum black and the brightest pixel to maximum white, and spreads everything else evenly between them. This however, may change the overall density of the image. Another control that may be available is Output Level. This sets the maximum black or white points that can be produced by, say, a printer. Generally, by setting the white point to at least 5 less than the maximum (in other words, to about 250)  you prevent the highlight areas in your output image appearing totally blank. Setting the black point to at least 5 more than the minimum (in other words, to about 5) you will help to avoid the shadow areas looking overly heavy in your output image.