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Although the practicalities of the development has been discussed, the processes involved need to be discussed.
In its most basic form, film is made by pasting photographic emulsion onto a transparent carrier. When the emulsion is exposed to light, the silver halide particles in the emulsion are made unstable by the light and form a latent image. The latent image is not visible under normal conditions, so must be enhanced, this is the function of the development process.
The developer translates the latent image by transferring the chemically altered
silver halide particles into black silver deposits. The more development the
film receives, the more black is formed until film that had been exposed to
very small amounts of light would be rendered black also.
The manufacturer provides a recommended film processing time, which indicate the normal development time for the film when used at the nominated film speed (ASA or ISO rating). To prevent the film over developing a stop solution is used which arrests the development process.
The fixing process actually prevents the emulsion from being reactive to light. Therefore the film can be handled in daylight once the fixing stage is complete. The secondary function of the fix is to make the un exposed emulsion into a transparent layer which allow light to pass through the negative. In addition to the clarification of the emulsion, the anti halation layer on the film is also cleared in the development process. If fixing is not completed correctly, the film appears cloudy, and can be put back into fix afterwards to finish, even after the wash cycle.