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The DSLR Field Guide by Michael Freeman

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Tone with Histograms and Levels

A histogram is a visual representation, in the form of a graph, of all the tones present in a digital image.

In digital photography, a standard 8-bit scale of 0–255 shows 256 columns from pure black at left (0) to pure white at right (255), and normally, in camera displays and Photoshop, they are packed together so that they join and there are no gaps. Pixel brightness is plotted across the bottom on the X axis while the number of pixels that contain a particular tone is plotted up the vertical Y axis.

At all stages of the photography workflow, this is the single most useful representation of the tonal qualities of an image. In Photoshop it appears as a palette (Windows > Histogram), and as an image-adjustment ...

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