Film developing
A piece of photographic film that has been exposed to light in a controlled manner must be developed before it can be used. Development does three things: it transforms the latent image into a visible image that can be seen, it makes the visible image permanent and resistant to deterioration with time, and it renders the film insensitive to light. Photographic paper is developed in a similar manner.
(This article does not apply to Polaroid film, which uses a self-contained developing process.)
The steps to black and white negative film developing are:
- The film is removed from the camera and wound onto a reel in complete darkness (usually inside a darkroom or a lightproof bag with arm holes). The reel holds the film in a spiral shape, with space between each successive loop so the chemicals may flow freely acrooss the film's surfaces. The reel is placed in a specially designed light-proof tank.
- The film is then treated with a series of chemicals:
- The first of these is developer, which makes the latent image visible.
- The second chemical is a stop bath, which stops the action of developer. (Sometimes plain water is used for this step.) It also helps to clean the developer residue off of the film to prevent contaminating the fixer.
- The third chemical is called fixer. It makes the image permanent and light-resistant.
- Fixer itself is slightly corrosive, so the film must be washed. Sometimes, it is then placed in a bath of a hypo clearing agent (and sometimes another material, such as selenium). It is then treated with a solution to prevent the appearance of hard-water stains, dried, and cut into strips of a convenient length.
In color negative processes the silver halide crystals present in the emulsion need to be removed. This is called bleaching. Bleaching may be done as an additional step or the bleaching agent, usually potassium ferricyanide, may be formulated in with the fixer. Bleaching may be skipped intentinally to create an optical effect on the film called Bleach bypass.