Developing the Film
When you deliver a roll of exposed film to the photo processor, it contains the latent images of the exposures that you made. These latent images must be amplified and stabilized in order to make a color negative that can then be printed and viewed by reflected light.
Before we cover the development of a color negative film, it might be best to step back and process a black-and-white negative. If you used black-and-white film in your camera, the same latent-image formation process would have occurred, except the silver-halide grains would have been sensitized to all wavelengths of visible light rather than to just red, green or blue light. In black-and-white film, the silver-halide grains are coated in just one or two layers, so the development process is easier to understand. Here is what happens:
- In the first step of processing, the film is placed in developing agent that is actually a reducing agent. Given the chance, the reducing agent will convert all the silver ions into silver metal. Those grains that have latent-image sites will develop more rapidly. With the proper control of temperature, time and agitation, grains with latent images will become pure silver. The unexposed grains will remain as silver-halide crystals.
- The next step is to complete the developing process by rinsing the film with water, or by using a "stop" bath that arrests the development process.
- The unexposed silver-halide crystals are removed in what is called the fixing bath. The fixer dissolves only silver-halide crystals, leaving the silver metal behind.
- In the final step, the film is washed with water to remove all the processing chemicals. The film strip is dried, and the individual exposures are cut into negatives.
When you are finished, you have a negative image of the original scene. It is a negative in the sense that it is darkest (has the highest density of opaque silver atoms) in the area that received the most light exposure. In places that received no light, the negative has no silver atoms and is clear. In order to make it a positive image that looks normal to the human eye, it must be printed onto another light-sensitive material (usually photographic paper).
In this development process, the magic binder gelatin played an important part. It swelled to allow the processing chemicals to get to the silver-halide grains, but kept the grains in place. This swelling process is vital for the movement of chemicals and reaction products through the layers of a photographic film. So far, no one has found a suitable substitute for gelatin in photographic products.
If your film were a color negative type (that gives you a print when returned from the photo processor), the processing chemistry is different in several major ways:
- The development step uses reducing chemicals, and the exposed silver-halide grains develop to pure silver. Oxidized developer is produced in this reaction, and the oxidized developer reacts with chemicals called couplers in each of the image-forming layers. This reaction causes the couplers to form a color, and this color varies depending on how the silver-halide grains were spectrally sensitized. A different color-forming coupler is used in the red-, green- and blue-sensitive layers. The latent image in the different layers forms a different colored dye when the film is developed.
- Red-sensitive layers form a cyan-colored dye.
- Green-sensitive layers form a magenta-colored dye.
- Blue-sensitive layers form a yellow-colored dye.
- The development process is stopped either by washing or with a stop bath.
- The unexposed silver-halide grains are removed using a fixing solution.
- The silver that was developed in the first step is removed by bleaching chemicals.
- The negative image is then washed to remove as much of the chemicals and reaction products as possible. The film strips are then dried.
The resultant color negatives look very bizarre. First, unlike your black-and-white negative, it contains no silver. In addition to being a color opposite (negative), the negatives have a strange orange-yellow hue. They are a color negative in the sense that the more red exposure, the more cyan dye is formed. Cyan is a mix of blue and green (or white minus red). The overall orange hue is the result of masking dyes that help to correct imperfections in the overall color reproduction process. The green-sensitive image layers contain magenta dye, and the blue-sensitive image layers contain yellow dye.
The colors formed in the color negative film are based on the subtractive color formation system. The subtractive system uses one color (cyan, magenta or yellow) to control each primary color. The additive color system uses a combination of red, green, and blue to produce a color. Your television is an additive system. It uses small dots of red, green, and blue phosphor to reproduce a color. In a photograph, the colors are layered on top of each other, so a subtractive color reproduction system is required.