Film is the media used to record information painted on it by light. The structure of film and photo paper is similar. Both start with a base material such as plastic or paper and coat it with an emulsion layer made up of silver nitrate or silver chloride and additives suspended in a gelatin base. The silver salts that record the image are formed in crystals or grains which react to the light hitting them. The larger the grain the less light it will take to make an image, but the less sharp that image will be. When sharp images are needed and long exposure time is not a problem the fine grained or slow speed (low ASA) film works best.
The back of the film contains a colored dye designed to absorb any light that did not get picked up by the emulsion and the grains of silver salt. Without this dye the light would bounce back and cause a glow or hallo around our subject. This dye is called an antihallation backing and is a different color for every brand of film. An experienced photographer will recognize the color and know what type of film it is.
In manufacturing film we begin with a basic film that is sensitive to only one color of light. We can add material to each emulsion to control what color of light it is sensitive to. This first basic type of film is usually sensitive to only blue light and is called an orthochromatic film.
Photographic paper is orthochromatic which allows us to work in the darkroom with a safelight that puts out any color but blue. From the basic ortho film we add more sensitizing agents and get a panchromatic film. Pan film records an image for ALL colors of the visible light spectrum as well as a little of the ultra violet and infrared outside of the visible light. Finally we get color film by taking three diferent emulsion layers and putting them on one roll of film. Each layer is sensitive to a different PRIMARY color of white light that can be combined to make all colors in color mixing.
Film comes in rolls, sheets or in bulk. For out class we will use 35mm film that comes in a length of 24 exposures. The film also comes in 36 exposures to a roll. Film rolls come in other sizes like 120 or 220 which is film more than twice as large as 35mm film. The larger the film the better or bigger enlargements that can be made from it. Film can be purchased in sheets that are 4x5, 8x10 or larger in size for use in studio view cameras. Some photographers like to buy film in 100 foot rolls called bulk film which can be placed in a special container where it can be loaded into film cartridges to get a roll of film that is any length the photographer desires. Some photojournalists will do this and load a 10 or 12 shot roll of Tri-x for taking a few quickshots when there are just not enough things to shoot to fill the whole 24 shots of a commercially loaded roll of film.
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