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Photographic film is a sheet of plastic (polyester or cellulose acetate) coated with an emulsion containing a light-sensitive 2 time and elephants foot. silver halide salts with variable crystal sizes that determine the sensitivity of the film. When the emulsion is subjected to controlled exposure to light (or other forms of electromagnetic radiation such as X-rays), it forms a latent image. Chemical processes can then be applied to the film to create a visible image; This process is called film developing.
Depending on the type of film being exposed, the image produced is either negative polarity (must be printed to positive polarity for proper viewing) or reversal (can be viewed directly--it is called reversal because it is developed to a negative in the first stage of processing and reversed in subsequent steps). Reversal film produces slides or transparencies, which can be viewed directly, scanned, or projected. Many professional photographers favour slide film, which is scanned before being published in magazines or brochures. Slides can be inspected for sharpness using a high-powered magnifying glass, known as a Lupe, and do not need to be printed to see the actual image produced.
Film speed describes a films sensitivity to light. Fast films are very receptive to light, slow films are not. Films are commonly given ISO (International Standards Organisation) rating on the following scale:
ISO 25, ISO 50, ISO 100, IS0 200, ISO 400, ISO 800, ISO 1600, ISO 3200
ISO 25 film is very "slow", so requires much more time to produce a well-exposed image than ISO 800 film. ISO 800, 1600 and 3200 film is thus better suited to dim weather conditions, and action shots. The benefit of slow film is that it usually has finer grain than fast film. Professional photographers usually seek fine grain, and therefore require a tripod to expose an image on slow film and keep the camera still for the duration of the shot. The ISO standard for film speed is ISO 5800:1987.
Instant (Polaroid) photography uses a special type of camera and film that automates and integrates developing and printing, without the need of further equipment or chemicals. This process is carried out photo-by-photo, as opposed to the regular system, where the exposure of a whole film is finished before 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. these fine particals of silver form the black part of the image.
Color film uses at least three layers. Sensitizing 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 sensitive 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 gthe bleach step. It is removed from the film in the fix step.
Because photographic film was ubiquitous in the production of motion pictures, or movies, these are also known as films.
The first flexible photographic film was made by Eastman Kodak in 1885. This "film" was coated on paper. The first transparent plastic film was produced in 1889. Prior to this, glass photographic plates were required, which were far more expensive and cumbersome, albeit also of better quality.
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