processed along with the production film and the gamma or contrast index is plotted When the gamma or contrast index remains constant, the processing is consistent. USING GAMMA AND CONTRAST  INDEX 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 70°F to a gamma of 0.90. Find 70°F and follow the horizontal 2-19