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Chapter Seven. Seeing the Light > Evaluating Exposure: Using the Histogram

Evaluating Exposure: Using the Histogram

The histogram is the primary tool for evaluating exposure, allowing you to check exposure either before or after (depending on your camera's features) you take the picture. This tool is incredibly useful for deciding whether or not you need to retake the shot with corrected exposure settings. Although the histogram is typically not offered on the least expensive digital cameras, it is available on all digital SLRs, many prosumer models, and even some deluxe point-and-shoot cameras as well. For serious image making with a digital camera, the histogram is an essential tool for making better pictures. In fact, on the few occasions that we do shoot film, we tend to find ourselves taking the picture and then looking at the back of the film camera—only to be disappointed not to see an LCD and histogram display.

The histogram shows how the tonal values in the image are distributed along the camera's grayscale tonal range. If you're new to the concept of a histogram, it's not really as mysterious as it seems. Essentially it's just a bar graph, with the horizontal axis representing the tones from total black on the left (level 0) to total white on the right (level 255). The vertical bars indicate the number of pixels at a given location on the tonal scale. With underexposed or dark images, the vertical bars tend to bunch up on the left side of the graph, while with photos suffering from overexposure, the vertical bars congregate on the right side. An exposure that exhibits a good tonal range with neither under- nor overexposure has data that covers the full range of the histogram (Figure 7.24).


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