“A camera is essentially a lightproof box with a hole, called an aperture, which is usually adjustable in size and regulates the amount of light that strikes the film. The aperture is covered with a lens, which focuses the image on the film, and a shutter, a kind of door that opens for a controlled amount of time, to regulate the length of time that the film is exposed to light- usually a small fraction of a second. Modern cameras also have a viewer that permits the photographer to see virtually the same image that the film ‘will see.’
Photography is based on the principle that certain substances are sensitive to light and react to light by changing value. In early photography, a glass plate was coated with a variety of emulsions; in modern black-and-white photography, film is coated with an emulsion of silver halide crystals (silver combined with iodine, chlorine or other halogens) suspended in a gelatin base. (Color photography uses a different light-sensitive emulsion).
The film is then exposed. Light reflected off objects enters the camera and strikes the film. Pale objects reflect more light than do dark ones. The silver in the emulsion collects most densely where it is exposed to the most light, producing a ‘negative’ image on the film. Later, when the film is placed in a chemical bath (developed), the silver deposits turn black, as if tarnishing. The more light the film receives, the denser the black tone created. A positive image is created from the negative in a darkroom; then the film negative is placed over a sheet of paper that, like the film, has been treated to be light sensitive, and light is directed through the negative onto the paper. Thus, a multiple number of positive prints can be made from a single negative.”
-Stokstad, Marilyn. Art History. Revised Second ed. Vol. 2. New York: Prentice Hall Inc., and Harry N. Abrams, 2005.