io to i : 5 ยฐ . The reproduction of tones by photography can be followed by means of the graphic solution designed by L. A. Jones (Communication No. 88). A photographer must consider three variables in the process: (I) The contrast of the subject ; (2) the contrast scale o f the negative, ~, ; (3) the contrast of the printing paper. The contrast of the printing paper is a complex value involving its exposure scale, its maximum black, and its 7.
In printing the paper is selected by the rule that the density scale of the negative is equal to the effective exposure scale of the paper. As a result of a statistical inquiry, L. A. Jones has found that the limiting gradients of a paper depend upon the negative on which the paper is printed, greater limiting gradient being required for printing a flat negative than a contrasty negative. The limiting gradient multiplied by the -/, of the negative is approximately a constant, this constant, however, depending on the density scale of the print.
The effective exposure scale of a paper is, therefore, dependent upon the 7 of the negative and thus upon the contrast of the subject chosen. The contrast of a paper is, therefore, the density range of the paper multiplied by the slope of the characteristic curve of the paper integrated throughout its length with regard either to exposure or to density. The application of tone reproduction theory to various branches of photographic practice is discussed briefly.