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Originally designed as a tool for instruction while Adams and Archer were teaching at the Art Center School in Los Angeles, Adams went on to popularize the method. While Adams' focus was on monochrome (black and white) photography using medium and large-format cameras, many elements of the method have also been applied to color and smaller formats, such as 35mm.
The zone system was developed in conjunction with the concept of visualization. (Adams often called this "previsualization," though he later noted that the term is a redundancy.) The finished print was visualized before the picture was taken, based upon the carefully measured luminosity range of the scene and an intimate knowledge of how film and print paper could render that scene.
Together, the zone system and visualization allow photographers to translate the light from a scene into specific densities on negatives and paper, thus giving them better control over the finished photograph.
The zone system separates the picture's tones into eleven zones, numbered using Roman numerals.
The zone system is chiefly used when exposing film, and more specifically, when deciding how to render shadow areas. Generally, the photographer wants to expose the film for the desired tone and amount of detail in the dark areas of the composition, then alter the development time to affect highlight density.
What makes the zone system so useful is the ease with which it translates desired effect to camera settings. Light meters will return the proper aperture ( f-stop) and shutter speed to expose the metered area at 18% middle grey - equal to zone V. Each zone above or below zone V corresponds to one f-stop. So, for example, if a photographer desires the shadow values of the ground below a bush to be a very dark tone, but still holding detail (zone III), he or she would close the aperture two stops from the light meter's reading of the shadow area. The smaller aperture results in less light hitting the film and thus in darker shadow areas.
Monochrome (black and white) film can record a light intensity range of about seven zones. The luminosity range of a scene, however, from the darkest shadows to the brightest areas, may be substantially greater or less than seven zones. Thus information can be lost in contrasty scenes due to the limited sensitivity range of film, or the range of film can be under-utilized in flat scenes.
The zone system manages this by:
The objective is to preserve and render detail the photographer feels esthetically important.
Black and white film once had a greater sensitivity range (50 years ago) because the emulsion was richer in silver, and thus able to hold more information. This changed when economics induced manufacturers to reduce the amount of silver.
Ansel Adams' famous book The Negative lays out preliminary calibration and darkroom tasks, taking test pictures of the gray card at different exposures, simulating the zones, and then developing the film. Following his directions will bring to light any miscalibration of equipment and variations of developer chemistry due to the local water supply. Once completed, the photographer will have correctly calibrated equipment and a solid understanding of the zones.
Film can be pushed (underexposed and overdeveloped) or pulled (overexposed and underdeveloped). This has the effect of expanding or contracting the sensitivity range recorded on the film, and can be applied to scenes that have a luminosity range unequal to the film. Pushing and pulling also affects the contrast of the final image, and the zone system helps the photographer to have full control over these techniques.
Color film has less range than monochrome, and sensors in digital cameras even less (although digital hardware is quickly improving). Some principles of the zone system, however, can be applied to color photography, largely in the area of light measurement and knowing what one can expect from color materials.