Photographic film is a material that is used in photographic cameras for recoding images. It is made of transparent plastic in a shape of a strip or sheet and it has one side covered with light-sensitive silver halide crystals made into a gelatinous emulsion. When a photographic film is exposed to light by a photographic camera, it chemically changes depending on the amount of light absorbed by each crystal. These changes create an invisible latent image in the emulsion, which is then fixed and developed into a visible photograph. Black and white photographic films have one layer of silver halide crystals while color film has three layers, each sensitive to a different color. Some color films have even more layers.
Early tries in photography didn’t use film but silver-plated copper sheets, paper and eve leather, covered with light-sensitive chemicals. Around middle of 19th century, glass plates became standard because early transparent plastics could not achieve opaqueness of glass and was still much more expensive than glass. The first film that was in a roll and flexible was made by George Eastman in 1885, but it wasn’t on synthetic but on paper. The first roll film on transparent plastic (on nitrocellulose which is highly flammable) was invented in 1889. "Safety film" was introduced by Kodak in 1908. It was made of cellulose acetate and was invented as a replacement for dangerous nitrate film. Nitrate film was much tougher, slightly more transparent, and cheaper and because of that “safety film” didn’t completely replaced it until 1951.
First Photographic plates that could produce images in color appeared in 1855 but they required complex equipment, long exposure times and were not too practical. Color films appeared in 1930s but these early films produced images that were too dark. In 1936 Kodak started selling Kodachrome which was much more similar to the modern color film because it used subtractive color method. This color film was used for home movies and for photographic cameras but was still too dark and much more expensive than black and white films. It needed some 40 years for color film to become standard and for black and white to be used for low-light photography and for art photography.
There are different types of films today:
Print films are standard, commonly used films. They produce transparent negatives (images with inverted colors) that are printed on photographic paper with use of an enlarger or by contact printing. These films can be color or black-and-white.
Color reversal film (or "slide film".) works the same as print film but it doesn’t invert colors. It makes diapositives (transparencies) which can be placed in small metal, plastic or cardboard frames and used as slides in projectors or slide-viewers. There are also color reversal sheet films of large format. They are used by professional photographers in order to produce very-high-resolution images which can be later scanned digitally.
Black-and-white reversal film exists but is used rarely. Positive black-and white slides can be produced from negative without much problems and this method is more commonly used.
Instant films are exposed and developed by instant camera after a photograph has been taken, without need to be developed in laboratory.
Some films are made to be sensitive to non-visible radiations: ultraviolet and infrared.
Those films made to be sensitive to X-ray radiation are used for medical imaging and can for instance see broken bones or foreign objects in body without need for invasive surgery.
Photographs used different types of films according to their characteristics. “Film speed” describes a film's sensitivity to light. “Slow” films have lower sensitivity and have to be exposed to light much longer to achieve same image as some “faster” films.