How Are the Pictures Formed on Camera Film?
Photographic film is a thin sheet of plastic, coated with special chemicals. Where light rays hit the film, chemical changes occur. When the shutter of a camera is snapped, light rays reflected from the scene shine on the film. The rays of light change the chemicals so that an image is formed on the film.
The image remains invisible until the film is developed. Other chemicals are used to develop the film, producing either a negative or positive transparency. Color film has a different kind of chemical coating from black and white film.
Film for cameras that use the 35 mm still format is sold as a long strip of emulsion-coated and perforated plastic spooled in a light-tight cassette. Before each exposure, a mechanism inside the camera is used to pull an unexposed area of the strip out of the cassette and into position behind the camera lens. When all exposures have been made the strip is rewound into the cassette.
After the film is chemically developed, the strip shows a series of small negative images. It is usually then cut into sections for easier handling. Medium format cameras use 120 film, which yields a strip of negatives 60 mm wide, and large format cameras capture each image on a single sheet of film which may be as large as 20 x 25 cm (8 x 10 inches) or even larger.
Each of these photographed images may be referred to as a negative and an entire strip or set of images may be collectively referred to as “the negatives”. They are the master images, from which all positive prints will derive, so they are handled and stored with special care.