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HANDBOOK OF PHOTOGRAPHY 



filter, which holds back both blue and red slightly, and, with artificial light, by the 

 darker Wratten X2 filter, which holds back more of the red. 



Average filter factors for these three types of panchromatic sensitizings and for 

 several degrees of orthochromatic sensitizing are given in Table IV. For most pur- 

 poses these are sufficiently accurate; it is only necessary to know to which group any 

 emulsion belongs. For very precise work, where very close matching of exposures 

 is necessary, the exact filter factors for any particular material can be obtained from 

 the manufacturer. However, since the filter factors depend on the spectral quality 

 of the light as well as on the emulsion, the values will hold only for light of the quality 

 specified. For light of anj^ other quality they will serve only as a rough guide. 



Table IV. — ^Filter Factors for the Types of Emulsion Sensitivity Illustrated 



IN Fig. 6 



Speed and Contrast. — The "emulsion speed" is a measure of the sensitivity of the 

 emulsion and determines the exposure required to produce the desired image. The 

 several methods of measuring speed differ greatly from one another; but, however 

 determined, a practical speed rating should indicate the relative exposure required to 

 produce a negative of good quality. For this, we do not need an absolute value but 

 only relative values which will give the ratios between the exposures required by 

 different films. 



The relative speeds of the negative emulsions vary over a range of over 100 to 1 for 

 daylight and even greater range for artificial light. Naturally, emulsions at the 



