The Histogram is the most important fingerprint of an image. It can warn you if an image is underexposed or overexposed. Modern cameras can display the histogram before you take the picture, so you can find the optimal exposure for the photo. The following examples show how to interpret histograms.
The left side of the curve represents the shadows, while the highlights are on the right side. If the histogram has a high peak on the left, a lot of pixels in the picture are dark, or in shadow. A peak on the right of the curve means that a lot of pixels are bright, or in the highlights. Peaks in the middle of the graph represent pixels in the midtones of your exposure. Ideally, curves blend into the horizontal axis before reaching the right and left edges.
Balanced exposure
A histogram, that starts from the horizontal axis on the left and the right side of the curve, can be considered well balanced. The light is well distributed through the image.
Underexposed photo
A histogram, that seems cropped at the left side, can be considered underexposed. This curve is missing values in the shadows, meaning that the picture is too dark.
Overexposed photo
A histogram, that seems cropped at the right side, can be considered overexposed. This curve is missing values in the highlights, meaning that the picture is too bright.
Critical photo
This photo can probably not be corrected. It was taken against the sun, so there is a very bright sunbeam in the photo, causing most of the rest to be badly underexposed. This can be deducted from the two peaks to the left and right side of the curve.
CodedColor has a number of tools to correct exposure. See the following topics for more infomation: