Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts that when exposed to light it forms a visible image. Through film developing, a chemical process applied to the film, a visible image is created. To create black and white photos there is only one layer of silver salts on the film and when exposed to light the silver salts block light and appear as the black part of the film.
Color film uses three layers of salts and dyes in order to create different colors. Normally the blue layer is on top followed by green and red. The salts react with color couplers that are included either in the film itself or in the develop solution. The silver salts are converted back in the bleach step then removed in the fix step leaving only formed color dyes which make up the colored visible image. New films, like kodacolor II, have as many as 12 emulsion layers with up to 20 different chemicals in each layer.
Since film has had such a long and wide use there are now around one trillion pictures on film or paper in world. The two types of film are print film which turns into a negative with the colors inverted and must be printed in order to view it as intended. The second type is color reversal film and can be viewed directly with a projector.
The film must be exposed properly in order to produce a usable image. The exposure latitude is the amount of exposure variation that a given film can tolerate and still produce an acceptable quality.
The optical density is the leftover concentration of dyes and silver salts on the film. A dark image has a higher density than a transparent image. Film is generally affected by physics of silver grain activation and from random grain activation by photons.
If exposed to the point of maximum density possible for print film then they will begin losing the ability to show tonal variations. These areas appear as featureless white when printed. The speed of film is its threshold sensitivity to light. ISO is the international standard for rating film speed. Consumer print is usually within the 100 to 800 range. ISO 25 is very slow and requires more exposure to produce a stable image while ISO 800 is much faster. Thus ISO 800 and greater is better suited for low light and action shots. However the higher the ISO the more grainy the picture usually is.
Eastman Kodak was the first to develop a flexible photographic film in 1885. The first transparent plastic film was produced in1889. Early films and plates were sensitive to blue light only until Hermann Wilhelm Vogel discovered that the spectral sensitivity could be extended by dye sensitization. In 1879 film sensitive to the spectral range from green to blue was introduced in 1879 and stayed dominant until the mid 1920s. This film and later film was used to produce black and white films regardless of spectral sensitivity. After World War II color photos were used overwhelmingly for the majority of photos.
Many different lenses and equipment are designed around the film to be used. Early lenses only needed to focus on blue light but the introduction of orthochromatic film required new equipment to focus from green to blue. The introduction of panchromatic film extended the need even further for specialized lenses and equipment. Many lenses used filters which were different for the different film types. A lens designed for orthochromatic film may have visible defects with a color emulsion.
Monochromatic film is processed in exactly the same way as a standard color film. There is also Polaroid film which is instant. Film can also record non-visible ultraviolet and infrared but require special equipment to do so. That equipment can range from a filter to a quartz lens.
Exposure and focusing are difficult when using UV or IR film since film speed only applies to visible light. A light meter is used to estimate exposure and the focus is slightly farther for IR and slightly closer for UV.
Film for X-ray radiation is used in medical imaging. For astrophotography a special film is used to attain long exposures.