CONTRAST INDEX
As you know, "contrast" means the range of
densities produced by a combination of subject
luminance ratio and the amount of development. The
luminance ratio of the scene cannot be controlled
outdoors and can be controlled only within limits in
the studio. When the degree of development is
controlled, one variable that affects negative contrast
is practically eliminated. When films are developed
to a given contrast index (CI) value, they can be made
compatible with any printing system. Variations in
contrast caused by different subject luminance ratios
are then adjusted in printing by selecting the
appropriate variable contrast printing filter. Controlled
film development helps to produce a uniform standard
of print quality, as well as to make printing easier and
less costly in both time and materials.
Contrast index has the same uses as gamma. It is
an index number that can be used for processing
control purposes. When films or developers are
compared, the test images should all be developed to
the same contrast index. Obtaining a constant contrast
index, like gamma, does not guarantee that all
negatives will have the same total contrast or that they
will all print similarly.
The contrast index is measured over that part of
the D-log H curve used in exposing continuous-tone
negatives correctly. CI is the average of the slope
and is distinctly different from the straight-line slope
of gamma. In this context, an average is the slope of
a straight line drawn between two definite points on
the curve. The straight line is drawn between two
points on the D-log H curve that represents the
highest (D-max) and the lowest (D-min) useful
densities on the characteristic curve.
To determine the contrast index, you must use a
transparent overlay on the D-log H curve. Refer to
figure 2-9 at this time. To use this transparent
contrast-index gauge, you must place it on the gross
fog density line of the characteristic curve. The gauge
is then moved right or left until the low-density arc
intersects the toe of the curve, and the high-density
arc on the shoulder of the curve reads the same value.
This value is the contrast index. Figure 2-9 shows the
proper use of a contrast-index gauge.
Since contrast-index gauges are not found in most
imaging facilities, an alternative method is used to
determine the contrast index of a characteristic curve.
This method is not as accurate as using a
contrast-index gauge, but provides an approximate
value. To use this method, first locate the density
point (in the toe area of the characteristic curve) that
is 0.10 above B+F. Then, using a compass, align it
on the log-H axis and spread it to a distance equal to
2.0 in logs. Place the point of the compass on the
density point that is 0.10 above B+F and draw an arc
on the curve. Finally, draw a straight line between
the two points and determine the slope, using the
same formula you used for gamma. The result is the
approximate contrast index.
CONTROL IN PROCESSING
The gamma or contrast-index value of a
photographic material is not fixed. Both vary,
depending on the process used. It is important for
you to realize that the contrast obtained by
development depends on the amount of development,
rather than on the development time alone. To obtain
an accurate gamma or contrast index, you must
control the total amount of development carefully,
such as developer temperature, developer strength,
degree of agitation, and other variables.
The densities of a characteristic curve are changed
by changes in development. Therefore, gamma and
contrast-index values are useful as a measure of the
degree of development. Gamma and contrast index
vary directly with the degree of development; the
greater the development, the higher the gamma or
contrast index. This is true until the film is grossly
overdeveloped. When film is grossly overprocessed,
the contrast begins to decrease, because the unexposed
silver halides have developed (increased gross fog)
after the maximum density has "peaked" out. The
point that the gamma or contrast index reaches its
maximum level is known as gamma (or CI) infinity.
Gamma and contrast index are two important tools
used in processing control. Films developed to the
same value, for example, show comparable tone
reproduction. When you want to determine whether
processing is consistent, sensitometric strips are
2-17

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