The Zone System is a technical approach to controlling negative contrast in order to match a subject brightness range to a particular paper.
The technique uses global adjustment of negative contrast though variations in development time to control dynamic range in the negative and achieve a printable image despite problematically high or low dynamic range in a scene.
Films and papers are first tested for their sensitivity (exposure required to produce the faintest shadow detail) and contrast using a step wedge and densitometer.
When shooting a scene, the film calibration information is used to select an exposure that will produce sufficient shadow detail and a development time that will achieve an appropriate contrast to fit the measured subject brightness range into the dynamic range of the the paper.
See also Beyond The Zone System.
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