Zone system

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The zone system is a system for determining the optimal film exposure and development, created by Ansel Adams. It is applicable, with minor modifications, to digital photography.

In the Zone system, Zone V is the midtone, identified by the exposure meter, with Zone X pure white and Zone 0 pure black. Each zone represents a change of one EV, or one stop.

• The full range from black to white is Zone 0 to Zone X
• The dynamic range is Zone I to IX, the darkest and lightest useful negative densities
• The textural range is Zone II to Zone VIII, being the ranges in which texture can be captured and displayed.

In use, the photographer picks the key element of the scene, and then decides which zone it should fall in. A meter reading, with a spot meter is then made, and the exposure is adjusted based on which zone the element should fall in. If the element were to be in Zone V, no adjustment would be needed, because the meter assumes it is measuring the midtone. If the photographer sees the key element as being in Zone III, the exposure is adjusted up by 2 EV from its reading, because Zone III is 2 EV below the midtone.

The reason for previsualising in this way is that averaging meters see tone in a very different way from the human eye. This is easy to see by taking two photographs of the horizon in the late afternoon, one directly under the sun, and one at about 90˚ to the right or left. The landscape directly under the sun will come out very dark by comparison with the exposure 90˚along the horizon. The human eye, though 'knows' the 'true' colours of objects, and compensates accordingly. Consider also a picture of a white stone in shadow, next to a black stone under strong sunlight. The human eye recognises that the white stone is significantly lighter than the shadow around it, and therefore registers it as 'white', and it registers the black stone as 'black' because it is significantly darker than the area around it. An averaging meter, however, is likely to give a similar level of grey to each one. The Zone system allows the photographer to see what the tone 'really' is, and to expose the image so that the element is recorded in that way.

The Zone system can be directly applied to digital photography, as long as the photographer remembers that digital is much more forgiving of underexposed shadow detail than of overexposed highlight detail. Therefore, the photographer should make a test exposure and check the histogram to ensure that the brightest value in the picture does not create a blown highlight, easily identified by a stack of values on the extreme right end of the histogram.