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.
Principles
Visualization
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 the 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.
Exposure metering
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.Adams (1981, 30) considered the incident-light meter, which measures light falling on the subject, to be of limited usefulness because it takes no account of the specific subject luminances that actually produce the image. 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 reflectanceA typical scene includes areas of highlight and shadow and has scene elements at various angles to the light source, so it usually is possible to use the term "average" reflectance only loosely. Here, "effective" average reflectance is used to include these additional effects. 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.
Exposure zones
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 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 10Adams (1981) designated 11 zones; other photographers, including Picker (1974) and White, Zakia, and Lorenz (1976), used 10 zones. Either approach is workable if the photographer is consistent in her methods. 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,Adams (1981) distinguished among exposure zones, negative density values, and print values. The negative density value is controlled by exposure and the negative development; the print value is controlled by the negative density value, and the paper exposure and development. Commonly, "zone" is also used, if somewhat loosely, to refer to negative density values and print values. 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,Photographers commonly refer to exposure changes in terms of "stops", but properly, a stop is a device that regulates the amount of light, while a step is a division of a scale. The standard exposure scale consists of power-of-two steps; a one-step exposure increase doubles the exposure, while a one-step decrease halves the exposure. Davis (1999, 13) recommended the term "stop" to avoid confusion with the steps of a photographic step tablet, which may not correspond to standard power-of-two exposure steps. ISO standards generally use "step". 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 (e.g. one f-stop) 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:
Zones as tone and texture
Adams distinguished among three different exposure scales for the negative: The full range from black to white, represented by Zone 0 through Zone X. The dynamic range comprising Zone I through Zone IX, which Adams considered to represent the darkest and lightest "useful" negative densities. The textural range comprising Zone II through Zone VIII. This range of zones conveys a sense of texture and the recognition of substance.
He noted that negatives can record detail through Zone XII and even higher, but that bringing this information within the exposure scale of the print is extremely difficult with normal processing.
Adams described the zone scale and its relationship to typical scene elements:Adams's description of zones and their application to typical scene elements was somewhat more extensive than the table in this article. The application of Zone IX to glaring snow is from .
For cinematography, in general, parts of the scene falling in Zone III will have textured black, and objects on Zone VII will have textured white. In other words, if the text on a piece of white paper is to be readable, light and expose the white so that it falls on Zone VII. This is a rule of thumb. Some film stocks have steeper curves than others, and the cinematographer needs to know how each one handles all shades of black-to-white.
Technique
Effective film speed
The International Standards Organization (ISO) standard for black-and-white negative film, ISO 6:1993, specifies development criteria that may differ from those used in practical photography (previous standards, such as ANSI PH2.5-1979, also specified chemistry and development technique). Consequently, the Zone System practitioner often must determine the speed for a particular combination of film, developer, and enlarger type; the speed determination is commonly based on Zone I. Although the method for determining speed for the Zone System is conceptually similar to the ISO method for determining speed, the Zone System speed is an effective speedThe effective speed determined for a given combination of film and developer is sometimes described as an "Exposure Index" (EI), but an "EI" often represents a fairly arbitrary choice rather than the systematic speed determination done for use with the Zone System. rather than an ISO speed.
Exposure
A dark surface under a bright light can reflect the same amount of light as a light surface under dim light. The human eye would perceive the two as being very different, but a light meter would measure only the amount of light reflected, and its recommended exposure would render either as Zone V. The Zone System provides a straightforward method for rendering these objects as the photographer desires. The key element in the scene is identified, and that element is placed on the desired zone; the other elements in the scene then fall where they may. With negative film, exposure often favors shadow detail; the procedure then is to: Visualize the darkest area of the subject in which detail is required and place it on Zone III. The exposure for Zone III is important, because if the exposure is insufficient, the image may not have satisfactory shadow detail. If the shadow detail is not recorded at the time of exposure, nothing can be done to add it later. Carefully meter the area visualized as Zone III and note the meter's recommended exposure (the meter gives a Zone V exposure). Adjust the recommended exposure so that the area is placed on Zone III rather than Zone V. To do this, use an exposure two stops less than the meter's recommendation.
Development
For every combination of film, developer, and paper there is a "normal" development time that will allow a properly exposed negative to give a reasonable print. In many cases, this means that values in the print will display as recorded (e.g. Zone V as Zone V, Zone VI as Zone VI, and so on). In general, optimal negative development will be different for every type and grade of paper.