202 HANDBOOK OF PHOTOGRAPHY 



then the basic concepts which have already been discussed for the proper tone rendition 

 on the negative apply equally well for the case of positive or printing materials. We 

 have, of course, to transfer the term "brightness range of the original subject" into 

 "opacity range of the negative," but otherwise the concepts are the same, and need not 

 again be gone through at this point. 



It is sufficient to call attention to the point that, in the ideal case, the exposure 

 range of the printing material must equal or exceed the opacity range of the negative 

 if we are to obtain a print which is an accurate, although reversed, reproduction of the 

 negative. Another necessary condition for ideal reproduction is that throughout 

 the exposure range a linear relation should exist between the logarithm of the exposure 

 and the density; in other words, we desire to work on the straight-line portion of the 

 Z)-logio E curve of the print as well as the straight-line portion of the D-logio E curve 

 of the negative. 



Unfortunately, this is not a practical solution because of the limited exposure and 

 density range of printing materials. As a result of the short ranges of density and 

 exposure, the toe and shoulder of the Z)-logio E curve are employed in most printing 

 applications; a print limited to the straight-line region of the characteristic curve 

 would very likely be flat (lacking in contrast) and would be suitable only for subjects 

 in which the brightness range (exposure range in the case of negatives) was decidedly 

 limited. 



Notwithstanding the fact that many papers have a very short linear region of the 

 D-logio E characteristics and certain papers may have no linear region, the concept 

 of the gamma, or slope of the straight-line portion of the curve, is a useful one to apply 

 in the case of printing or positive materials. To distinguish the characteristics of the 

 positive or printing materials from that of negative materials, we shall use the symbol 

 7p to designate the gamma of the positive material, and 7„ to designate the gamma of 

 negative materials. This distinction between the slope of the characteristics of the 

 two types of photographic materials will be especially useful when we come to consider 

 the proper rendition of tone in the entire photographic process. 



Because the Z)-logio E curves of many printing papers have a relatively short 

 linear region, the concept of jp is not so thoroughly intrenched as that of 7^ for negative 

 materials. Manufacturers do not specify the limiting values of 73, which may be 

 expected to obtain upon prolonged development, nor do they specify the density range 

 or the exposure range from which some conception of yp might be derived. 



Printing papers are available having various values of exposure range and density 

 range, and these factors do give in some measure an indication of the gamma of the 

 printing material under consideration. However, papers are not specified by the 

 manufacturers according to their sensitometric characteristics, except in a somewhat 

 roundabout mystical manner in which vague terms are employed. With sufficient 

 exposure, the density range of a printing material is determined, to a very considerable 

 degree, by the texture of its surface. The exposure range of papers is usually specified 

 by the manufacturers by such vague terms as "soft," "vigorous," or "hard," in 

 decreasing values of exposure range, or by some numerical system. Such designations 

 are hardly satisfactory from the sensitometric point of view. And yet the system 

 has worked apparently quite well. There seems to be little inclination on the part of 

 manufacturers to employ a more rigorous specification for their printing materials so 

 long as the average photographer does not demand more specific data. 



Even though we maj^ not know the actual H and D characteristics of the printing 

 material we are employing, the concept of the D-logio^ curve is a useful one. 

 Through its use, we may prepare a chart of printing-paper characteristics, so far as 

 this relates to the accurate reproduction of the "image" of the negative. Such a set 

 of curves, as is shown in Fig. 29, is based on the assumption that we wish to reproduce, 



