618 



HANDBOOK OF PHOTOGRAPHY 



when the characteristics of three-color printing pigments and inks are taken into 

 consideration. 



The characteristic transmission curves of the Wratten three-color filters Nos. 25, 

 47, and 58 are given in Fig. 3. 



The presence of a color filter in the path of the light reduces the total amount of 

 light reaching the photographic emulsion and therefore a longer exposure is necessary. 



1.8 



1.5 



1.2 



^ 



c 0.9 



0.6 



0.3 



12 3 4 5 



Steps o-^ Neu+ral Gray Wedge 

 LogiQ Exposure Me+er- Candle-Seconds 



Fig. 4. — Typical characteristic curves of panchromatic emulsions. Curve B represents the 

 characteristic of the film when exposed through a green filter. 



This increase in exposure is called the "filter factor," and it is ordinarily given by the 

 manufacturer of the photographic material. In the case of three-color filters the 

 factor pertaining to each filter is of such value as to permit obtaining the same general 

 results when all three negatives are developed all together for the same length of time. 

 For accurate results color-separation negatives should be developed to the same 

 contrast, which, as we shall see immediately, requires different developing times. 



Luminosity range 



2 4 8 



No. 1. 



Fig. 5.- 



16 



No. 2 No. 3 No. 4 No. 5 



-Gray scale or neutral-tint wedge. 



Therefore a slight readjustment of the filter factor is desirable for color-separation 

 negatives to be correctly processed. 



A typical characteristic curve of a panchromatic emulsion is given in Fig. 4. 



The curve is subdivided into three well-known regions: underexposure, the use- 

 ful region, and the region of overexposure. 



The necessitj'- of correct exposure in color photography will be immediately evident 

 from the following considerations. 



Let it be assumed that a neutral-tint wedge, Fig. 5, is placed alongside the subject. 



