The histogram is a graph which represents the distribution of light or luminance values in your image. Its horizontal axis ranges from black and dark shadows on the left to bright or pure white on the right. Although there is no 'perfect' distribution for a histogram, a well-exposed image will have luminance values that fall within the histogram's range for your camera, with no pixel's luminance values falling on the extreme left (shadows recorded as black) or on the extreme right (highlights recorded as white), unless you are trying to achieve a particular creative result. Fortunately, all modern digital cameras can display an histogram, which you should check early and often to help ensure that you achieve a good exposure. To view the histogram, preview your image on the LCD screen of your camera, and flick through the options until a graph appears (some cameras only show luminance, while others also show the Red, Green, and Blue channel values). If it shows values on either extreme, you may want to adjust your aperture, shutter speed, or ISO, and then retake the image to regain that lost information.
If the brightness values in the scene fall outside of you Histogram's range (i.e., outside of its dynamic range) even after making appropriate exposure adjustments, then you may want to properly expose for the shadows and highlights in separate images, and then combine or blend those exposures in an image-editing software program like Adobe Photoshop or Photomatix; this technique is also called HDR, or High Dynamic Range photography.
Special thanks to Wen Zhang: www.flickr.com/photos/transcendent
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