COLOR PHOTOGRAPHY 



625 



The color-separation negatives of still-life subjects indoors can be made by succes- 

 sive exposures with an ordinary camera rigidly mounted to preclude any chance of 

 movement while changing negative holders and color filters. The filters can be slipped 

 onto the lens in the usual manner. Care must be taken that the three film or plate- 

 holders register correctly. 



Several devices to shorten the total time required for the triple exposure have been 

 invented, among which outstanding is the repeating back by Miethe. 



Colour Photographs, Ltd., London, has introduced the fully automatic repeating 

 back, in which the sliding of the plateholder and the operation of the shutter is accom- 

 plished automatically in accordance with a predetermined setting. 



A repeating back in its simplest form is obtainable from the Autotype Company, 

 London. The three-color-separation filters of a repeating back are placed imme- 

 diately in front of each negative. 



The most satisfactory method of obtaining color-separation negatives by simul- 

 taneous exposure is hj subdividing the light emitted from the lens by means of partial 

 reflecting mirrors. 



Single-mirror Camera. — The simplest form of 

 camera designed on this principle is the one involv- 

 ing the use of a single partial reflecting mirror and 

 shown schematically in Fig. 13. To obtain three- 

 color-separation negatives with this type of camera, 

 it is necessary to expose two of the negatives in 

 bipack arrangement. The location of the bipack ^'' 

 in the camera depends entirely on the nature of the 

 partial mirror. The following important points 

 must be carefully considered in designing the 

 camera: 



1. The commercial bipack available today is 

 composed of an orthochromatic emulsion in front 

 and a panchromatic emulsion in the back. The 

 two emulsions are facing each other, but the front 

 emulsion has an additional layer of gelatin carrying a 

 reddish filter dye. If the bipack is exposed through a magenta or minus green filter 

 (Wratten No. 32), it yields a blue-sensation negative (front emulsion) and a red- 

 sensation negative (back emulsion). If, on the other hand, the bipack is exposed 

 through a yellow filter (Wratten K3) it yields the green-sensation negative (front 

 emulsion) and red-sensation negative (back emulsion). Under this condition the 

 sensitivity of the red negative overlaps somewhat too much into the green. 



In any case the red negative, which gives a positive in blue-green or cyan color, 

 is decidedly diffused due to the separating layer of gelatin carrying the red filter and the 

 light-scattering effect of the front emulsion. The blue-green color of the positive 

 print is the most important color for assuring the definition of the whole picture, and 

 therefore the use of the standard bipack for this type of camera should be avoided if 

 possible. 



2. The commercial tripack can be split up for use with the single-mirror camera in 

 two parts by removing the back emulsion to be exposed separately. The two front 

 emulsions exposed as bipack, but without any filter, yield a sharp blue-sensation 

 negative (front emulsion) and a very slightly diffused green-separation negative 

 (second emulsion). Since a yellow filtering dye is incorporated in the first emulsion 

 without any extra layer of gelatin, the contact between the two front emulsions is 

 much closer than in the case of the standard bipack, and the resulting green-separa- 

 tion negative is sufficiently sharp for all practical purposes. 



Fig. 13. — Diagram illustrat- 

 ing optical system of single- 

 mirror camera. 



