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

Figure 2-9.-Using a contrast-index gauge.

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.