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