film – Darkroom Color Printing – Filter changes and exposure times


If you set higher and higher values ​​for one of the three dichroic glass filters, a correction is required (filter factor that increases the exposure). I can give approximate values, but there are many things you need to do your own testing for. Exposure meters are available. However, you can use almost all light meters that are on the base bar. If you choose more filtering, the light level will be reduced.
Choose in .10 = 1/3 stop
Choose in .30 = 2/3 stop
Select .50Y = 1 stop

Why dichroic? All filters fade over time, but magnifiers are particularly bad due to the bright light and high heat. We started color printing with colored gelatin (CC-Filter Color Correcting) and acetate (CP-Filter Color Printing). As these quickly fade in the lamp house, we turned to dichroic glass. A dichroic filter is made by depositing micro-thin layers of metals or oxides on refractory glass. These work according to the interference principles like the rainbow that you see on soap bubbles. In any case, the metal or oxide layers are inert. Dichroic filters reflect heat and the colors of light they do not pass, so they run much colder than the mass-colored cousin. For example, a yellow dichroic substance is reflected blue and yellow by transmitted light, etc. (twice Greek). A magenta-dichroic looks magenta through transmitted light and green through reflected light. A cyan dichroic looks red by the transmitted light and by the reflected light.

When we print a color negative film with the enlargement lens wide open, the paper is struck by an abundance of all three bright primary colors (red – green – blue). We stop the magnifying lens to reduce this light intensity. If you continue, the red light energy will be correct first. This is due to the ISO allocations (actually paper speeds). At the right red exposure energy, we address the green and blue light energy, both remain in excess.

Now we introduce a green blocking filter. This is the magenta filter. We choose the magenta value until the green light energy is correct. Next we choose a blue notch filter. This is the yellow filter. In this way, we set the exposure of the paper / negative combination so that the paper receives correct red, green and blue exposures. Red again is achieved by dimming the lens, green is achieved by adjusting the magenta filter, and blue by adjusting the yellow filter.

This scheme avoids the use of the cyan filter. We avoid, because the cyan filter is difficult to produce and all cyan filters let unwanted frequencies through. The yellow filter is almost a perfect blue blocker. The magenta is OK, but not perfect. The cyan filter is so bad that it has a high filter factor, as we have to dial in more and more to get the job done. This leads to crosstalk. Use the iris diaphragm better to control the red energy level. Thus, the paper layer speeds are adjusted so that this scheme can be used.