PHOTOGRAPHIC MATERIALS 



143 



emulsions the contrast can be expressed in terms of the slope of the characteristic 

 curve or of the density difference obtained for a given range of exposure or range of 

 object brightness. Because of the length of the straight-line portion of the curve, it is 



1.5 



0.3 







2.4 2.7 3.0 



0.3 0.6 09 1.2 1.5 1.8 2.1 

 Log Exposure, Log Ey 



Fig. 15. — Z)-logio E characteristics for a typical chlorobromide paper. 



not difficult to obtain theoretically correct tone reproduction in the negative. With 

 papers, on the other hand, the straight-line portion of the characteristic curve is com- 

 paratively short, and, except when special effects are wanted, it is necessary to make use 

 of the full density range, which is determined by the paper surface and type of emul- 



> 



0.5 06 



09 



2.4 



2.7 3.0 



1.2 1.5 1.8 2.1 

 Log Exposure, Log Ey 



Fig. 16.^ — £)-logio E curves for a typical bromide paper for development times which increase 

 in alphabetical order. The characteristics resemble those of negative emulsions. 



sion. Thus it is not possible to obtain perfect reproduction over the whole range of 

 tones. However, if the extreme shadow tones are ignored, the negative exposure can 

 be so chosen that, with the proper combination of materials, the curvature in the 



