The contrast of photographic printing paper

Abstract

The results obtained in Section C may be summarized as follows: 1. (a) A careful analysis of the errors involved in the sensitometry of positive materials has been made from which it is concluded that the reading error is of greatest magnitude. The total error, however, is small, and a characteristic curve based upon the average of five sensitometric strips should not be subject to a density error of more than ±.015. 2. (b) The effect of development time upon the characteristic curve of the positive material is discussed. It is shown for these, with the exception of the bromide papers, that the development is very rapid. Maximum density and maximum gradient are obtained in a relatively short time, after which further prolongation of development produces no change in the shape of the characteristic curve, but does produce a lateral shift of the entire curve. From the standpoint of contrast, therefore, an equilibrium is reached after a relatively short time of development. It is pointed out that the contrast of a material must depend upon time of development unless development is carried to such an extent that equilibrium in the shape of the curve is reached. 3. (c) Results of the sensitometric examination of each of the thirty-seven positive materials used are given. It is shown that from the coördinate values of four critical points all of the necessary sensitometric constants, with the exception of D max., can be computed. This method gives some additional sensitometric constants and results in a more complete expression in numerical terms of the characteristics of the material as expressed by the density-log exposure function. 4. (d) Using the sensitometric curve for each material and the values of maximum and minimum density for the optimal prints, the color coefficient of the negatives is computed. This method gives a value in very close agreement with direct determinations of color coefficient by strictly sensitometric methods.