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252 PRINCIPLES OF CINEMATOGRAPHY
image spread or contraction. From the figure it is seen that, in this example the lowest response, that of minus 50 decibels, is produced when a negative having a density of 1 -9 is printed so that a positive having a density of 1 -4 is produced. Although the process necessary to produce this result is somewhat lengthy, it need only be carried out to establish these optimum conditions at the outset when new recording equipment and processing techniques are installed. Once these densities have been established the normal sensitometric methods may be used to maintain the laboratory at these conditions.
It is interesting to compare the densities recommended by the classical sensitometric methods with those just given. The sensitometric negative density normally recommended is 2-10, whereas that indicated by cross modulation in this example is 1 -90. Referring to Figure 113, and assuming that the print will be made to a density of 1-4, the sensitometric recommendation would result in a low frequency modulation 15 decibels greater than that produced by the negative density recommended by the cross modulation method. If, on the other hand, the positive print density is increased to 1 • 5 when using the sensitometric recommendations, this difference falls to 5 decibels. However, it must be remembered that it has only been possible to observe this degree of correction, which can be obtained by changing the positive density, after the cross modulation tests have been made and the curves in Figure 113 have been plotted.
It may at first be thought that the advantages gained by this process do not warrant the initial work which is involved. This is certainly not true, particularly since the best curve in Figure 113 has been found to be relatively steep. Providing that the positive density is maintained at 1-4, the negative density may vary between 1-8 and 2-0 without any serious disadvantage but, immediately the negative density goes beyond these limits, the high frequency wave forms become considerably distorted owing to image spread or contraction. This point is shown by the rapidity with which the low frequency output increases on each side of the trough of the 1-4 positive density curve in the figure.
The application of Cross-Modulation analysis to practical work has resulted in the recognition of the term 'cancellation density'. This refers to the practice of intentionally slightly over-exposing the negative and then slightly under-exposing the positive print which, it has been found, will result in a considerable improvement in image sharpness without materially affecting the optimum overall condition.