the straight line intersects the scale. As you can see, the gamma for the plot is 0.82. CONTRAST 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, D D   (difference   in   density)   may   be considered the negative contrast (for the straight line) and D 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. 2-16