For black-and-white ground pictorial photography, a gamma of about 0.65 to 0.90 or a contrast index of 0.56 to 0.60 is adequate for most printing systems. Some printing systems, such as those using specular light sources, require a lower CI. For these systems, a lower value may be more suitable. The best gamma or contrast index for a particular printing system can be determined only through practical tests. These tests can be made by developing several equally exposed films to different gamma or contrast-index values. The negative with a gamma or CI value that prints best on your printing system is then used. When using variable contrast paper, you should aim for negatives that print well with a No. 2 filter.

Time-Gamma and Time-Contrast Index Curves

Time-gamma or time-contrast index curves are plotted to summarize the behavior of any film-developer combination. Time-gamma or time-contrast index curves are used to indicate the time of development required to reach a desired gamma or contrast index. Time-gamma and time-contrast index charts can be made to show the maximum gamma (gamma infinity) or maximum contrast index (contrast index infinity) obtainable with a given film-developer combination.

Figure 2-10 shows a time-gamma curve and the family of curves that produced it. A small graph is drawn in the upper left comer of the graph on which the family of curves is plotted. The horizontal axis of the small graph indicates time of development in minutes and the vertical axis indicates gamma or contrast index, as appropriate. The gamma or contrast index obtained from each sensitometric strip of the film being tested is plotted against the time required to produce it by placing dots in their proper position on the graph. A french curve is used to connect the dots.

To use the curves, select the required gamma or contrast index from the vertical column of the small graph, and read the development time needed from the times given just below the base line, or horizontal axis. These times should be based on negatives made under average conditions. If the negatives subsequently developed are too low in contrast, choose a higher gamma, or contrast-index value. When the contrast is too high, choose a lower value.

The processing latitude is the range in times of development for any given tolerance in gamma or contrast index. The processing latitude may be found by determining the minimum and maximum gammas or contrast indices that are acceptable.

By examining the time-gamma or time-contrast index curve, you will notice that as development increases, gamma or contrast index also increases. This increase is rapid at first and then increases more slowly. After a period of time, there is little increase in gamma or contrast index, even though development is prolonged This indicates that for any particular emulsion and development condition, the higher the gamma or contrast index, the greater the processing latitude. This also indicates that the lower the gamma or contrast index, the more precise processing conditions must be to obtain uniform development.

Time-Temperature Charts

One of the primary factors affecting the amount of development and the formation of density of an image is the temperature of the developer. The higher the temperature, the greater the activity of the solution. As the temperature drops, the developing time must be increased. Since gamma or contrast index must also be considered, typical time-temperature charts include a gamma and/or contrast-index value that varies according to development time and temperature. By consulting a time-gamma or a time-contrast index temperature chart, you can determine the proper developing time under varying conditions. These charts are published by film manufacturers and can be seen throughout the Photo-Lab Index. Figure 2-11 is a typical time-gamma chart. To use a time-gamma or time-contrast index temperature chart, follow the line indicating the temperature at which you are processing until the desired gamma or contrast-index line is intersected. From this point, drop straight down to the time of development line. The intersection of the vertical line and the time of development line indicates the proper developing time at the recommended agitation. For example, using figure 2-11, assume the film is to be processed at 70F to a gamma of 0.90. Find 70F and follow the horizontal

Fiure 2-10.-Time-gamma and family of curves.

Figure 2-11.-Time-gamma temperature chart.

line until it intersects the bold line labeled .9 (0.90 gamma). From this point, drop straight down to the time of development line and read 7.5. It takes 7 1/2 minutes of developing time at the recommended agitation.