While photography is a mainstay of modern life, and most people are at least somewhat aware of the fact that film can be developed in a darkroom, the process of film development itself is less widely understood. While there are many different methods of developing film, they all rely on a number of chemicals.
Black and white processing
The process for developing black and white film is almost the same as developing color film, and is made up of the same basic steps. It is, however, somewhat simpler than color film development; with black and white film, every part of the image is developed in the same color. Developing all film requires three chemicals: the developer, the stop bath, and the fixer.
The developer does just that: develops the film. While the image has been captured on the film, it is not visible until the developer brings out the silver halides in the film. On the negative, brighter parts of the picture will remain dark, while the darker parts of the picture will be lighter. Common chemicals used as developing agents are hydroquinone, phenidone, and dimezone. The developing mix must have high acidity, so chemicals such as sodium carbonate or sodium hydroxide are often added to the mix.
While the developer will bring out the image captured on the film, it will continue to develop the picture until it is stopped. This will ruin the film, and is what is referred to as overexposure. So, once the film has been developed to an appropriate degree, the developer is replaced with a stop bath. The stop bath neutralizes the developer and stops the film from becoming overexposed. A common chemical used in stop baths is acetic acid. While the developer could also be washed off with water, using acetic acid is both quicker and more thorough.
The photographic fixer is the final process in developing the film. The fixer "fixes" the image in place by removing the unexposed silver halides in the film. This process stops the film from further reacting to light, and thus stops the sensitive chemicals in the film from being altered. Common fixing chemicals are ammonium thiosulfate and sodium thiosulfate.
The chemical process involved in developing color film is similar to the black and white process. The difference is in the developing stage. A chemical consisting of paraphenylene diamine is used to develop the film. When the film is added to this color developing bath, the paraphenylene diamine exposes not only the fiilm's silver halides, but also what are called "dye couplers," which are chemicals in the film itself that are made of different colors.