Abstract

A large amount of experimental data has been obtained to determine what portion of the D-log E curve of photographic paper is useful in making high quality prints. Psychophysical statistical methods were necessary in solving this problem. Special attention was given to selecting a procedure which would yield results of maximum practical significance. Prints were made from about 200 negatives using five different exposures on each of three contrast grades of paper for each negative. Most of the work was done with a normal, commercially available set of semi-matte surface papers having six different contrast grades. Additional work was done using glossy and matte surface papers. Every set of prints was judged by a number of observers to determine the best print. The location of the “first-choice” prints on their respective D-log E curves was then obtained from maximum and minimum density readings on the prints and on the negatives. The data for second-choice and third-choice prints were also obtained as a matter of general interest. It was found that the minimum or shadow density of a negative is a much more reliable guide for determining correct printing exposure than the maximum or highlight density. The integral or average density of the negative is nearly but not quite as satisfactory for this purpose as the minimum density. Of great interest is the fact that the useful maximum density of the printing paper was, on the average, considerably below the available maximum density, while the useful minimum density of the paper was usually very close to, and often equal to, zero density. The primary requirements, however, appear to be expressed by gradients rather than by densities. It was found that the limits of the useful portion of the D-log E curve of the positive material are determined very closely by fractional gradients of 0.1 Ḡ on the toe of the curve and 1.0 Ḡ on the shoulder of the curve, where Ḡ is the average gradient measured between these two limiting points. This conclusion has an important bearing on the establishment of a significant method for expressing the sensitometric characteristics of photographic papers. It leads to an evaluation of the effective printing speed and the useful exposure scale of the positive material. The proposal is made that speed be defined as 10⁴/E, where E is the exposure corresponding to the limiting gradient on the shoulder of the curve. The useful log exposure scale of the paper is defined as the log exposure interval lying between the two limiting gradients. The application of these results to the making of prints by sensitometric control is discussed. In general, a negative should be printed so that its minimum density falls on the shoulder of the D-log E curve at the point where speed is measured. A method is given whereby the density scale of the negative can be used with reasonable success to determine the proper contrast grade of paper for making the print.

© 1942 Optical Society of America

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