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The histogram

The histogram measures the quality of the exposure of an image. It is the most important metric available to the digital photographer.

Recall that a digital image consists of a collection of numbers, each with three components for red, green and blue, respectively. Each of the RGB components has a value in the range (usually) of 0 to 255.

The histogram is generated by counting the number of occurrences of 0, 1, 2, etc. up to 255, in the image. These 256 counts are then plotted as the Y axis versus 0 to 255 on the X axis.

The example at right is the Levels dialog box in Photoshop for the image below it.

The tonal range of the image is immediately apparent, with shadows on the left, midtones in the middle, and highlights on the right.

The histogram of an image with an ideal exposure has wide dispersion of tones declining to zero at the extremes.

Blowout of highlights is indicated by a vertical line at the right edge.

DSLRs have the ability to display histograms of stored images, a very useful way to check exposure in the field. Under- or over-exposure is readily apparent.

 Pluses and Minuses of Digital SLRs            © Robert Harrington 2004