A camera histogram is a chart in a digital camera that evaluates the exposure of each shot. A graphical representation of each exposure shows balances in brightness in a similar manner to a bar chart. Reading a camera histogram involves looking at the levels throughout the chart and noting any indications of overexposure or underexposure. Basic uses of histograms help photographers make adjustments to prevent shots that are washed out or too dark. A color histogram measures balances of the different color channels in a photo and can help prevent over-saturation of a certain hue or shade.
The standard digital camera histogram measures the value of each individual pixel's brightness value. Higher bars in a histogram frequency distribution indicate a greater number of pixels of a given value. The histogram is generally divided into three separate horizontal sections according to brightness. The far left section indicates shades of black, the middle section is for mid-gray shades, and the far right one is for varying degrees of white. Each area of brightness value also corresponds to the 256 tones available for rendering digital photographs — zero represents pure black and 255 indicates pure white.
Histogram analysis for a correct exposure usually depends on the type of exposure effects photographers hope to achieve. A normal exposure without a high amount of contrast has a camera histogram reading with pixels evenly distributed across the range of shadows, mid-tones, and light areas. Pictures with a higher degree of contrast have histograms with bars clustered at both ends of the chart with few pixels in the middle. Lower-contrast images have pixel bars that are clustered in the middle histogram area. Some advanced digital cameras have settings that allow photographers to include the camera histogram in the preview screen so that it changes according to the given subject matter and provides a handy guide for using correct lighting.
An additional photography histogram indicates the numbers of colored pixels in an image. This information is normally available as separate camera histograms of each color, usually shades of red, blue, and green. Large amounts of pixels in each color channel indicate higher color saturation in varying degrees of brightness. A steep peak on the left section of a blue histogram indicates a greater amount of deeper blue in a given picture. Many photographers find that color camera histograms help them make better choices for both lighting and image editing.