When color positive printing paper is used, things work just the opposite. With color positive printing paper, when red light strikes the paper, less cyan dye is produced. The more red light, the less cyan dye that will be formed. The more green light, the less magenta dye that will be formed. The more blue light, the less yellow dye that will be formed.

For purposes of photography, all white light contains only RGB wavelengths of color. In a dichroic (subtractive) enlarger, some of the RGB wavelengths of light are removed (subtracted) from the rest of the light that reaches the color printing paper. The subtracting is done by partially inserting "subtractive" filters into the beam of white light before the light passes through the color negative.

If a cyan filter is partially inserted into the beam of white light coming from the light bulb in a dichroic enlarger, it will stop some of the red wavelengths from getting past it. It "subtracts" red light. This means that less red light will be able to reach the photographic printing paper.
If a magenta filter is partially inserted into the beam of white light coming from the light bulb in a dichroic enlarger, it will stop some of the green wavelengths from getting past it. It "subtracts" green light. This means that less green light will be able to reach the photographic printing paper.

If a yellow filter is partially inserted into the beam of white light coming from the light bulb in a dichroic enlarger, it will stop some of the blue wavelengths from getting past it. It "subtracts" blue light. This means that less blue light will be able to reach the photographic printing paper.

If all three filters (CMY) are used, a little of all of the light will be stopped from reaching the photographic paper. This reduction in the intensity of the enlarger's output is referred to as having reduced the intensity by about one f/stop. Because of manufacturing inaccuracies, it will require slightly different settings on each of the three filters in order to produce pure neutral density. For example, it might require settings of something like 29C - 31M - 27Y. Each enlarger will require something slightly different.

Since most folks don't want to cause a reduction in the output, usually only two of the enlarger's three filters are used at any one time. When printing onto color negative paper, the cyan filter is usually not used. If the cyan filter is not going to be used, there are two other ways to control the amount of red light that reaches the printing paper.

The easiest way is to simply make a change in the lens f/stop or in the setting of the exposure timer, then adjust the magenta and yellow filters as needed. But, since a slight shift in the color balance of the paper will occur every time there is a change in the exposure time, most folks use a second method of controlling the red/cyan color balance in a print.

Here's how the second method works. If a test print appears to have too much red color in it, there is actually too little cyan dye formed in the emulsion. If a test print appears to have too much cyan color in it, there is actually too much cyan dye formed in the emulsion. The amount of cyan dye (too much or too little) is always judged in relation to the amount of the other two dyes that might be present--magenta and yellow.

So, if you make an equal amount of real change in the other two dyes (magenta and yellow) there will appear to be a change in the cyan dye. Therefore, if you use equal amounts of the magenta and yellow filters, it will appear to cause a change in the red/cyan color balance of color in the print. Using less magenta and yellow filtration will cause an apparent decrease in the cyan color in the print. Whenever the color cyan decreases in a print, the color red has to increase and by an equal amount, and vice versa. The colors, red and cyan, have a teeter-totter relationship.

All dichroic enlargers are nothing more than tools that provide a means of controlling the amount of RGB light that reaches the color printing paper.

If you'd like more help with your darkroom activities, you can contact me by sending e-mail to: editorial@shutterbug.net.

Photographic Paper
Color paper is made up with only three dyes: cyan, magenta, and yellow. By overlapping these three colors in various combinations all of the other colors are created. When red, green, and blue light from the enlarger strikes photographic paper it stimulates the production of the three dyes. A dichroic enlarger controls how much red, green, and blue light reaches the photographic paper by means of subtractive filters that are partially inserted into the beam of light from the enlarger's bulb. The cyan filter subtracts some of the red component of the light. The magenta filter subtracts some of the green component of the light. The yellow filter subtracts some of the blue component of the light.