Although digital cameras are wildly popular, many photographers still prefer to use film. The film format is the basically the size of the film. The common sizes available are 35mm, APS, 120 and 4"x5" sheet film. Larger sheet film and subminiature films are also available, but some older sixes are no longer made. There are various types, and speeds of film. Each type and speed of film is basically designed for certain situations. For example, fast film such as 1600 ISO is made for action and low light photography.
Film also comes in different exposures, such as 12, 24 and 36. Film is basically a sheet of plastic, polyester, celluloid or cellulose that is coated with an emulsion that contains light-sensitive, silver halide salts. The sensitivity or resolution of film is determined by variable crystal sizes. When the emulsion is subjected to controlled light it creates an image which becomes visible when certain chemicals are added to the film. This is called film developing.
There are two main types of photographic film; these are print film, which turns into a negative with the colors or black and white values inversed. This film must be transferred to photographic paper to be viewed. There is also reversal or slide film, which is called a transparency after developing. This can be viewed directly by a loupe or projector. Prints can be made from reversal film, but it is expensive. Black and white reversal film does exist, but it is uncommon. Professional photographers prefer reversal film because it provides better color reproduction.