Editing Zone System (section)

== Technique ==

=== Effective film speed ===

The 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&nbsp;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 speed''<ref>
The 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.
</ref>
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&nbsp;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&nbsp;III. The exposure for Zone&nbsp;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&nbsp;III rather than Zone&nbsp;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&nbsp;V as Zone&nbsp;V, Zone&nbsp;VI as Zone&nbsp;VI, and so on). In general, optimal negative development will be different for every type and grade of paper.

It is often desirable for a print to exhibit a full range of tonal values; this may not be possible for a low-contrast scene if the negative is given normal development. However, the development can be increased to increase the negative contrast so that the full range of tones is available. This technique is known as ''expansion'', and the development usually referred to as "plus" or "N+". Criteria for plus development vary among different photographers; Adams used it to ''raise'' a Zone&nbsp;VII placement to Zone&nbsp;VIII in the print, and referred to it as "N&nbsp;+&nbsp;1" development.

Conversely, if the negative for a high-contrast scene is given normal development, desired detail may be lost in either shadow or highlight areas, and the result may appear harsh. However, development can be reduced so that a scene element placed on Zone&nbsp;IX is rendered as Zone&nbsp;VIII in the print; this technique is known as ''contraction'', and the development usually referred to as "minus" or "N−". When the resulting change is one zone, it is usually called "N&nbsp;−&nbsp;1" development.

It sometimes is possible to make greater adjustments, using "N&nbsp;+&nbsp;2" or "N&nbsp;−&nbsp;2" development, and occasionally even beyond.

Development has the greatest effect on dense areas of the negative, so that the high values can be adjusted with minimal effect on the low values. The effect of expansion or contraction gradually decreases with tones darker than Zone&nbsp;VIII (or whatever value is used for control of high values).

Specific times for N+ or N− developments are determined either from systematic tests, or from development tables provided by certain Zone System books.

=== Additional darkroom processes ===

Adams generally used [[selenium toning]] when processing prints.  Selenium toner acts as a preservative and can alter the color of a print, but Adams used it subtly, primarily because it can add almost a full zone to the tonal range of the final print, producing richer dark tones that still hold shadow detail. His book ''The Print'' described using the techniques of [[dodging and burning]] to selectively darken or lighten areas of the final print.

The Zone System requires that every variable in photography, from exposure to darkroom production of the print, be calibrated and controlled. The print is the last link in a chain of events, no less important to the Zone System than exposure and development of the film. With practice, the photographer visualizes the final print before the shutter is released.