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.
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.
2-11, assume the film is to be processed at 70F to a
gamma of 0.90. Find 70F and follow the horizontal
2-19

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