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HANDBOOK OF PHOTOGRAPHY 



negative just balances the curvature in the positive, giving practically perfect repro- 

 duction over the greater part of the tone scale. ^ 



The range of negative tones which can be reproduced in the print is determined 

 by the paper-exposure scale, or the ratio between the exposure which will just produce 

 a visible density and the least exposure which will produce the maximum black. 



Since negatives made by amateurs are made under widely varying conditions of 

 lighting, and consequently of subject brightness, and since development conditions 

 are not always carefully controlled, amateur negatives vary widely in density range. 

 Therefore printing papers for amateur use are usually supplied in a number of grades 

 of contrast, with exposure scales varying from about 1 : 5 for the extra-contrast papers, 

 to as much as 1 : 50 for the very soft papers. This range of contrasts is covered in five 

 or six steps, so that it is possible to get acceptable prints from negatives having density 

 ranges between 0.7 (opacity range of 5 to 1) and 1.7 (opacity range of 50 to 1) (Fig. 17). 



I 2 4 8 16 32 64 



Exposures 



Fig. 17. — Relative contrast of six types of printing paper popuarly used in amateur 

 photography. The slope of the curve is a measure of contrast, whereas the distance from 

 unity to the projection on the abscissa is a measure of the relative exposure range obtainable. 



The professional photographer's negatives are usually made with controlled light- 

 ing and controlled conditions of development, and so have a comparatively uniform 

 density range. Therefore papers for professional use are often made in only one 

 or two degrees of contrast, corresponding approximately to the "normal" grade of the 

 amateur papers. 



Although all the paper-emulsion types can be made in a range of contrasts, the 

 chloride or gaslight papers tend in general to have the shortest exposure scales (most 

 contrasty), and the bromide papers tend to have much longer exposure scales. The 

 chlorobromide papers are, in general, intermediate. 



Although paper contrast could be expressed by a numerical value, derived possibly 

 from the exposure scale, photographers in general do not have the means for measuring 

 the density ranges of their negatives in order to make use of such values. Therefore 

 the various contrast grades have been designated by such descriptive terms as "soft," 

 "medium," "normal," "hard," "vigorous," etc. This works fairly satisfactorily, 

 though there is the disadvantage that, since the terms are only generally descriptive, 

 different manufacturers may use one term for papers of considerably different exposure 

 scale. 



1 Jones, L. A., The Evolution of Negative Film Speeds in Terms of Print Quality, J . Franklin 

 Inst., 227, 297 (March 1939). 



