Histogram

The histogram is a graph showing the tonal value from pure white (on the right of the graph) to pure black (on the left). This is a very useful tool for evaluating your photos. There is no ideal shape to the histogram but large spikes at the extreme edge are to be avoided. A histogram showing these spikes at the edges means some areas aren't properly exposed; spikes at the right mean details are over exposed or "clipped", while those on the left mean areas are under exposed. Isolated areas of over/under exposures are not always undesirable; that can add drama to the photo.
Histogram photo

Here is a photo after editing, showing good exposure. There are no over or under exposed areas.

Note: the reduction in quality of this photo for the web has resulted in the photo loosing a good deal of its contrast

Camera Histogram
This is a photo of the histogram as shot. Taken by itself, this histogram would seem to indicate an underexposed photo (highest peaks to the left). but the shaper's table is rather dark, as are parts of the dust collection hood, the upper left corner of the background, and the opening around the cutter head. The exposure has kept detail in the very light curly maple, the highlights on the dust collection hood, and the top edges of the fence. Keeping details in the highlights, with corresponding underexposure of the shadows is usually preferable to having blown out highlights.
finished histogram
This is a shot of the Levels histogram in Photoshop Elements 7, after some very minor manipulation. A close look at the finished photo and the photo as shot will show this minor editing brought out a little more detail in the shadows, (especially the shadow cast by the fence on the table) without any hint of over exposing the highlights.

 

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