624 



HANDBOOK OF PHOTOGRAPHY 



The negatives exposed behind the green and blue filters are developed in exactly 

 identical manner with the developer renewed each time. 



When all the negatives are dry, they can be measured in the densitometer and the 

 values thus obtained plotted on cross-section paper with the density of the wedge on 

 the horizontal axis and the density of the negative on the vertical axis. 



Three families of curves are thus obtained, one for each filter. The value of gamma 

 or contrast for each curve is the ratio of the intercept on the horizontal axis to the 

 intercept on the vertical axis by the straight line passing through the straight section 

 of the curve and prolonged to meet the horizontal and the vertical axis. 



Figures 10, 11, and 12 are typical tests conducted on Dupont panchromatic films. 



Densities of NEGATrv'E 



1.0 



0.5 



2.0 



1.5 :^- 



c 



(U 



O 



1.0 a, 



> 



'+- 

 C 



0.5^ 



Fig, 



2.0 1.5 1.0 05 



Wedge Densi+y 



12. — Characteristics of panchromatic material exposed through blue filters. 



It is obvious from the time-gamma curve that in order to obtain a gamma equal 

 to 1 with the Wratten three-color filters and ABC pyro developer at 68°F., the 

 red-, green-, and blue-separation negatives must be developed 11, 11, and 20 min., 

 respectively. 



The blue-filter negative of a given panchromatic emulsion usually requires a 

 longer development time to build up the same gamma as the other two negatives. 

 This phenomenon can be partially explained by the fact that blue light penetrates 

 less into the depth of the emulsion as compared with light of longer wave length.^ 



In color photography the most satisfactory type of negative is one which gives a 

 good print on a normal type of bromide paper. This corresponds to approximately 

 unity gamma, and such value should be adopted as standard with great advantage. 



Cameras and Equipment for Exposing Three-color Separation Negatives. — Before 

 entering into discussion of color printing processes it will be convenient to review the 

 type of cameras and various equipment for exposing three-color-separation negatives. 

 The types of cameras here described are those in which each color-separation negative 

 sees the total cone of light emerging from the lens. 



These types are therefore free from the so-called "parallax" error which occurs 

 when each color-separation negative covers only a zone of the exit pupil of a lens. 

 Cameras with parallax errors are also defective from the standpoint of even distribu- 

 tion of light throughout the area of negative. 



1 An exception to this rule is the new Ilford Trichrome plate which builds up the same gamma irre- 

 spective of the filter. Therefore three color-separation negatives made with this type of plate are devel- 

 oped for the same length of time. 



