The Zone System is a photographic technique for determining optimal film exposure and development, formulated by Ansel Adams and Fred Archer. Adams described the Zone System as "[...] not an invention of mine; it is a codification of the principles of sensitometry, worked out by Fred Archer and myself at the Art Center School in Los Angeles, around 1939–40."
The technique is based on the late 19th century sensitometry studies of Hurter and Driffield. The Zone System provides photographers with a systematic method of precisely defining the relationship between the way they visualize the photographic subject and the final results. Although it originated with black-and-white sheet film, the Zone System is also applicable to roll film, both black-and-white and color, negative and reversal, and to digital photography.
An expressive image involves the arrangement and rendering of various scene elements according to the photographer's desire. Achieving the desired image involves image management (placement of the camera, choice of lens, and possibly the use of camera movements) and control of image values. The Zone System is concerned with control of image values, ensuring that light and dark values are rendered as desired. Anticipation of the final result before making the exposure is known as visualization.
Any scene of photographic interest contains elements of different luminance; consequently, the "exposure" actually is many different exposures. The exposure time is the same for all elements, but the image illuminance varies with the luminance of each subject element.
Exposure is often determined using a reflected-light exposure meter. The earliest meters measured overall average luminance; meter calibration was established to give satisfactory exposures for typical outdoor scenes. However, if the part of a scene that is metered includes large areas of unusually high or low reflectance, or unusually large areas of highlight or shadow, the "effective" average reflectance may differ substantially from that of a "typical" scene, and the rendering may not be as desired.
An averaging meter cannot distinguish between a subject of uniform luminance and one that consists of light and dark elements. When exposure is determined from average luminance measurements, the exposure of any given scene element depends on the relationship of its reflectance to the effective average reflectance. For example, a dark object of 4% reflectance would be given a different exposure in a scene of 20% effective average reflectance than it would be given in a scene of 12% reflectance. In a sunlit outdoor scene, the exposure for the dark object would also depend on whether the object was in sunlight or shade. Depending on the scene and the photographer's objective, any of the previous exposures might be acceptable. However, in some situations, the photographer might wish to specifically control the rendering of the dark object; with overall average metering, this is difficult if not impossible. When it is important to control the rendering of specific scene elements, alternative metering techniques may be required.
It is possible to make a meter reading of an individual scene element, but the exposure indicated by the meter will render that element as a medium gray; in the case of a dark object, that result is usually not what is desired. Even when metering individual scene elements, some adjustment of the indicated exposure is often needed if the metered scene element is to be rendered as visualized.
In the Zone System, measurements are made of individual scene elements, and exposure is adjusted based on the photographer's knowledge of what is being metered: a photographer knows the difference between freshly fallen snow and a black horse, while a meter does not. Much has been written on the Zone System, but the concept is very simple—render light subjects as light, and dark subjects as dark, according to the photographer's visualization. The Zone System assigns numbers from 0 through 10 to different brightness values, with 0 representing black, 5 middle gray, and 10 pure white; these values are known as zones. To make zones easily distinguishable from other quantities, Adams and Archer used Roman rather than Arabic numerals. Strictly speaking, zones refer to exposure, with a Zone V exposure (the meter indication) resulting in a mid-tone rendering in the final image. Each zone differs from the preceding or following zone by a factor of two, so that a Zone I exposure is twice that of Zone 0, and so forth. A one-zone change is equal to one stop, corresponding to standard aperture and shutter controls on a camera. Evaluating a scene is particularly easy with a meter that indicates in exposure value (EV), because a change of one EV is equal to a change of one zone.
Many small- and medium-format cameras include provision for exposure compensation; this feature works well with the Zone System, especially if the camera includes spot metering, but obtaining proper results requires careful metering of individual scene elements and making appropriate adjustments.
Zones, the physical world and the print
The relationship between the physical scene and the print is established by characteristics of the negative and the print. Exposure and development of the negative are usually determined so that a properly exposed negative will yield an acceptable print on a specific photographic paper.
Although zones directly relate to exposure, visualization relates to the final result. A black-and-white photographic print represents the visual world as a series of tones ranging from black to white. Imagine all of the tonal values that can appear in a print, represented as a continuous gradation from black to white: