A negative contains several different contrasts. It has "total contrast," defined as the difference in density between the useful shadow and highlight densities in a negative. Total negative contrast is a useful index to determine what contrast printing filter to use. Total negative contrast is dependent upon factors, such as subject luminance ratio, camera exposure level, color of light, and gamma. Gamma is only one of the factors that controls the total contrast of a negative. In addition, a negative has other forms of contrast as follows:

Shadow contrast-associated with the toe section of the characteristic curve and, therefore, unrelated to gamma.

Highlight contrast-associated with the shoulder of the characteristic curve and gamma does not apply.

Midtone contrast-associated with the straight-line section of the characteristic curve. GAMMA

Gamma, therefore, is not a measure of negative contrast, nor does controlling the gamma in different negatives ensure that they will all print with the same contrast printing filter. All of the frames on a roll of film may be developed uniformly to the same gamma; for example, 0.75. However, each negative probably has a different total contrast because of variations in subject luminance ratio, subject color, lighting, and so forth. "Contrast," therefore, is best defined as a range of densities produced by a combination of the subject luminance ratio and the amount of development given a film.

Film exposed on a cloudy bright (no shadows) or heavy-overcast day could be developed to a high gamma (for example, 1.6) and still produce a flat negative, because of the small luminance range of the subject being photographed. Another scene with a high-luminance ratio could be photographed and developed to a low gamma of say 0.50, yet the contrast of the negative could be so high that it requires a low-number contrast printing filter.

You should understand that even though a negative can have high-total contrast, there may be little contrast in the shadows when those shadows fall on the flat portion of the toe. When two films with the same characteristics are exposed to the same scene, at the same time, and each film is developed to a different gamma, more contrast can be expected in the negative developed to the higher gamma. This is true for those tones exposed on the straight-line section of the characteristic curve and to a lesser extent for tones exposed on the upperpart of the toe.

When the basic formula for determining gamma is examined, AD (difference in density) may be considered the negative contrast (for the straight line) and A log H (difference in log H) the subject contrast. Gamma can then be considered as the ratio between negative and subject contrast. A negative that is developed to a gamma of 1.00 has, for all straight-line exposures, the same contrast range as the original scene. When the negative is developed to a lower gamma (for example, 0.50), it has only half as much contrast as the subject. Remember, this applies only to the straight-line section of the curve.

Gamma, however, is not always appropriate for measuring the effects of exposure and development. Gamma does not take into consideration that the toe of the curve is normally used for recording shadow tones in ground pictorial, continuous-tone film. Also, D-log H curves for different films have different toe lengths and toe shapes; consequently, film developed to a given gamma may not yield a uniform density range sufficient enough for ordinary continuous-tone photography. To provide a more uniform density range, you can use a form of averaging the gradient, called contrast index. However, in some applications where the characteristic curve has a long straight-line region and the image is recorded totally on the straight-line section of the curve, gamma is still a valid method of measuring density range.

In aerial photography, for example, it is desirable to record shadows, midtones, and highlights on the straight-line section of the characteristic curve. When all subject tones are recorded on the straight-line section of the curve, the greatest amount of tone separation is obtained in all areas of the image (shadows, midtones, and highlights). This provides the maximum amount of detail in all areas of the negative. A greater emphasis is placed on detail, rather than a "pretty picture," in most aerial photographic applications.