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Photographic film is a sheet of plastic used to record pictures. The plastic has been specially treated, so it is sensitive to light. That way, it is possible to record an invisible image with the plastic when it is exposed to light. Normal film is shipped in small canisters (boxes) that protect it from the light. A normal photographic film can hold up to 40 pictures. Once all pictures have been recorded, the film has to undergo a special chemical treatment. This is called developing a film. Through that treatment, the pictures become visible (you can see them), and the exposed film is no longer sensitive to light.
Different kinds of films exist. Some require more light to be exposed than others. Some are black and white only; they record no colors. There are also special films which can record infrared light.
Film can only be used once. After that, it cannot be used again (if it is accidentally used again, this results in an artifact called a multiple exposure). When not in use, the film needs to be covered from light, otherwise it will record any lights that shine on it. This will make it useless to record a picture later on. Film comes in a can called a canister to cover it from light rays.
Film needs the right amount of light to make a picture. If the picture is too bright or too dark, it will not record correctly. The longer that the film keeps recording, the more light it will get. If what is being photographed is bright, it will be recorded faster. If it is darker, the film will need more time to record.
Films that need less time to record the picture are known as "faster" films. Different speeds of films are marked with an ISO number. The higher the number, the faster the films. Usual ISO numbers for films are 100, 200 and 400. Films can only make a picture from focused lights. If there is no lens to focus light, the film will only turn white from receiving just the light. If a film with an ISO level of 200 instead of 100 is used, it will only need half as much time to record a picture of the same scene.
Films also come in different sizes. 35 millimeter film, the most used size, comes in metal cans or canisters, but there are other camera films that come in paper wrappings or in single sheets.
Before photographic film was invented, photography used glass plates.