Effective film speed/Exposure/Development

Effective film speed
The ISO standard for black and white negative film, ISO 6:1993, uses chemistry, development technique, and development criteria that often have little relation to those used in practical photography (the same was true for previous standards). Consequently, the Zone System practitioner often must determine the speed for a particular combination of film and developer; 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 speed 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. 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 a 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.It often is 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 VII placement to Zone VIII in the print, and referred to it as “N + 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 IX is rendered as Zone 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 - 1” development.It sometimes is possible to make greater adjustments, using “N + 2” or “N - 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 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