Reducing Light or Tone of Digital Photos

On the other hand, let's take the (b) definition of color as white light minus a zone of the visible spectrum. Subtractive means it reproduces tones by subtracting from white light (from a projector) the regions of the spectrum whose absence defines a color. A slide or a color negative, once developed, can be considered as a variable color and density light filter that affects a same beam of white light and perfectly describes the color of a scene. Therefore the operation a subtractive system uses is subtraction , understood as the alteration of light through an interposition of filters. Earlier we have seen three color definitions with this system: yellow is white light minus blue, magenta is minus green and cyan minus red. On this same basis we can affirm that:

Red is white light minus green and minus blue (minus cyan).
Green is white light minus red and minus blue (minus magenta).
Blue is white light minus red and minus green (minus yellow).

And, therefore:

red and cyan,
green and magenta,
blue and yellow

are complementary to each other, because en each pair the primary color represents a third of the spectrum and the secondary is the sum of the other two thirds. Each one is what is missing from the other to be white light. In practical terms, saying “too much green” is the same as saying “too little magenta”, “taking away blue” is equal to “adding yellow”, “more red” is equal to “less cyan”, and vice versa. In color film, the basic color proportions in each point of the scene are regulated by

different densities of yellow, magenta and cyan pigment, because starting from these colors we can reproduce any range of tones on a same support. From three black and white negatives that separately register red, green and blue, pigment densities are developed of their corresponding complementary colors. Each pigment layer retains a quantity of light of it's basic complementary color in correspondence with the intensity of said color in each point of the image. Since the filter used to regulate each third of the spectrum is of a color that is transparent for the other two thirds, it affects exclusively one of the basic colors without altering the other two. White light, when passing through three superimposed layers, looses red, green and blue in amounts that depend on the density of the pigments in each point.

We have revealed the mystery of color film: when projecting a slide, when we go to the movies or look at a color negative, we are seeing an image constituted of three layers of yellow, magenta and cyan pigments. The manufacturers and researchers that have dedicated large efforts to the development of this process have had to overcome, one by one, technical difficulties of enormous complexity: discovering a way to sensitize a black and white emulsion to each one of the primary colors; develop a system to superimpose the three emulsions without it affecting the correct registration of each color; once the film is exposed, developing the three images that have registered the information of it's respective color in each point of the image; associate each black and white image with it's corresponding complementary color pigment in a way proportional to the density, eliminate the silver image and give stability to the obtained image.