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PROCESSING MOTION PICTURE FILM 127
Characteristic Curves
Curves produced by plotting exposure values against resultant densities are sometimes known as 4H & D' curves, after Hurter and Driffield, the two scientists who first employed this method of representing photographic characteristics. Obviously, it would be of little value to deal with Opacities and actual Exposure times when plotting these curves since the scales of reference would be very widely spaced at one end and very crowded together at the other. However, by taking logarithms of each scale, an evenly distributed calibration is obtained throughout the total range of densities and exposures. When logarithms of Opacities are plotted, as shown on the vertical scale in Figure 55, they are termed 'Densities'. When logarithms of the actual exposure times are plotted, as shown on the horizontal scale, they are known as 'Log Exposures' this axis being popularly known as the 'Log E' axis.
Some people find considerable difficulty in plotting characteristic curves because they are never quite sure of the relationship between the units of density and the units of log-exposure. We have seen that the sensitometer gives a series of exposures, each of which is 1 -4142 times greater than the exposure preceding it. It will also be seen from Figure 55 that the logarithms of the exposures increase as follows: 0 -15, 0 -30, 0 -45, 0 -60, 0 -75, and so on, that is, each step increases by 0 -15. If, for example, we let 0-15 increase in Log-Exposure be represented by 15 divisions along the graph paper, we must also let 0 -15 increase in Density be represented by 15 divisions since, in actual fact, we are plotting one logarithm against another. Confusion always arises because the density scale is marked off not in units of 0 *15, but in units of 0 -10 under such circumstances it must be remembered that the ratio between the two scales is still maintained although different intervals are numbered on each scale. In short, no matter what type of graph paper is used, but providing the exposure increments are known to be V2, the number of divisions along the graph paper which are used to represent a change of 0 -1 in Density must always be 2/3 the number used to represent one exposure step. It should be noted that, although <^2 is actually 1 -4142, the logarithm of this is 0 -15.
Assuming that a range of sensitometric exposures has been made, processed and measured, a curve similar to that shown in Figure 56 will be produced. It will be noticed that a section of this curve, from point 'B' up to point 'C\ will be substantially straight and will, therefore, indicate that within this range, densities will be produced in proportion to the magnitude of the