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Photographic film is a sheet of plastic (polyester, celluloid (nitrocellulose) or cellulose acetate) coated with an emulsion containing light-sensitive silver halide salts (bonded by gelatin) with variable crystal sizes that determine the sensitivity and resolution of the film. more...
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When the emulsion is subjected to sufficient exposure to light (or other forms of electromagnetic radiation such as X-rays), it forms a latent (invisible) image. Chemical processes can then be applied to the film to create a visible image, in a process called film developing.
In black-and-white photographic film there is usually one layer of silver salts. When the exposed grains are developed, the silver salts are converted to metallic silver, which block light and appear as the black part of the film negative.
Color film uses at least three layers. Dyes added to the silver salts make the crystals sensitive to different colors. Typically the blue-sensitive layer is on top, followed by the green and red layers. During development, the silver salts are converted to metallic silver, as with black and white film. The by-products of this reaction form colored dyes. The silver is converted back to silver salts in the bleach step of development. It is removed from the film in the fix step. Some films, like Kodacolor II, have as many as 12 emulsion layers, with upwards of 20 different chemicals in each layer.
Because photographic film is widespread in the production of motion pictures, or movies, these are also known as films.
Film basics
There are two primary types of photographic film:
Print film, when developed, turns into a negative with the colors (or black and white values, in black and white film) inverted. This type of film must be "printed" — either projected through a lens or placed in contact — to photographic paper in order to be viewed as intended. Print films are available in both black-and-white and color.;
Color reversal film after development is called a transparency and can be viewed directly using a loupe or projector. Reversal film mounted with plastic or cardboard for projection is often called a slide. It is also often marketed as "slide" film. This type of film is often used to produce digital scans or color separations for mass-market printing. Photographic prints can be produced from reversal film, but the process is expensive and not as simple as that for print film. Black and white reversal film exists, but is uncommon — one of the reasons reversal films are popular among professional photographers is the fact that they are generally superior to print films with regards to color reproduction. (Conventional black and white negative stock can be reversal- processed, to give 'black & white slides', and kits are available to enable this to be done by home-processors. As indicated by Grant Haist's published book 'Modern Photographic Processing', B&W transparencies can be produced from most all B&W films.;
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