This information is something of a repeat of some messages I posted a few years ago.
From the turn of the century until the early 1950s one-shot color cameras were manufactured that made three exposures at one time, one through the Red filter, the other through a Green filter, and a third through a Blue filter. This was achieved by means of pellicle beam splitters that transmitted a part of the light but reflected the other part. Before that cameras with sliding backs were used without the pellicle beam splitters.
It is very feasible to make color separations with B&W film, making exposures on three different sheets of filmn with a view camera. For in-camera separations the recommended filter set is 25 Red, 58 Green and 47 Blue. You mark the film holders as R, G or B, and before developing clip the R on one corner, the Green on two corners and leave the B as is. This helps to identify them later. You can combine the separation negative files, after scanning the negative, to combine the spectrum for a full color shot. If you use an Apo lens and choose stationary subjects the quality can be quite high, at least as good as with transparency film of the same size.
The best film for color separations is one that has a short toe and shoulder and a very long straight line. Both Tmax-100 and TMY work quite well. For best results you will need to expose and develop the three records individually, but if you have the luxury of curve correction in Photoshop just give +3 stops exposure for the Red and Green filters, and +2 stops for the Blue filter. Develop the Red and Green negatives for the same time, the Blue for about 50% more.
There may be faster ways of doing this but this is how I merge the separation layers in Photoshop.
1. Name the scanned files by the taking filter, Red Negative, Green Negative and Blue Negative.
2. Choose File>Scrips>Load Files into Stack. Check auto align box.
3. Convert the separation files to their respective subtractive colors. Change the Red negative file to cyan, the Green negative file to magenta, and the Blue negative file to yellow.
A. Convert the files to their color.
B. First, select the Red negative file. Then go to Windows>Swatches and put the eyedropper tool over the CMYK Cyan patch, located in the upper left area. Click on the CMYK Cyan patch and the foreground color will change to Cyan. Now go to Edit>Fill and make sure Foreground Color is selected and that Blending Mode is set to screen.
C. Repeat for the Green negative file, choosing CMYK Magenta with the eyedropper tool to set the foreground color.
D. Repeat for the Blue negative file, choosing CMYK Yellow with the eyedropper tool to set the foreground color.
4. Combine the three separate images to form a full color image.
A. Create a new file, File>New. This file should be at least as large as the largest of the three separation layers. Set Mode to RGB and set background to White.
B. Copy the first layer to the new file. In the old file make the Red/Cyan layer the active layer, choose Select>All, then Edit>Copy Merged. Now go to the new file and do Edit>Paste.
C. Repeat for the Green/Magenta layer and for the Blue/Yellow layer.
D. When finished the Blue/Yellow layer should be on top. Make this layer active in the Layers Palette and change the blending mode to Multiply.
E. Repeat this step for the Green/Magenta layer.
F. Leave the blending mode set to Normal for the Red/Cyan layer
G. Your image should now be in color. Save the file.
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