Photography

Photography, use of light sensitive material to produce permanent visible images (photographs). In the most familiar processes a photographic emulsion is a preparation of tiny silver halide crystals suspended in a thin layer of gelatin coated on a glass, film, or paper support. On brief exposure to light in a camera or other apparatus, a latent image in activated silver salt is formed wherever light has fallen on the emulsion. This image is made visible in development when the activated silver halide crystals (but not the unexposed ones) are reduced to metallic silver (black) using a weak organic reducing agent (the developer). The silver image is then made permanent by fixing with agents that dissolve out the silver halide crystals that were not activated on exposure. The image is a “negative” one. To produce a positive image, the negative is itself made the original in the above process, the result being a positive “print,” usually on a paper carrier. An alternative method of producing a positive image is to bleach away the developed image on the original film or plate before fixing, and reexpose the unactivated halide in diffuse light. This forms a second latent image that on development produces a positive image of the original subject (reversal processing).

The history of photography from the earliest work of Joseph Nicephore Niepc, Louis Daguerre, and Fox Talbot to the present has seen successive refinements in materials, techniques, and equipment. Photography became a popular hobby after Eastman first marketed roll film in 1889.

Motion-picture photography dates from 1890, when Thomas Edison built a device to expose Eastman's roll film. It rapidly became an important art form. Not all modern photographic methods employ the silver halide process; Xerography and the blueprint and ozalid processes work differently. False-color photography and the diffusion process used in the Polaroid Land camera are both developments of the silver halide process.