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Photography, technique of producing permanent images on sensitized surfaces by means of the photochemical action of light or other forms of radiant energy.

In today's society, photography plays important roles as an information medium, as a tool in science and technology, and as an art form, and it is also a popular hobby. It is essential at every level of business and industry, being used in advertising, documentation, photojournalism, and many other ways. Scientific research, ranging from the study of outer space to the study of the world of subatomic particles, relies heavily on photography as a tool. In the 19th century, photography was the domain of a few professionals because it required large cameras and glass photographic plates. During the first decades of the 20th century, however, with the introduction of roll film and the box camera, it came within the reach of the public as a whole. Today the industry offers amateur and professional photographers a large variety of cameras and accessories.

Basic Technology

Light is the essential ingredient in photography. Nearly all forms of photography are based on the light-sensitive properties of silver-halide crystals, chemical compounds of silver and halogens (bromine, chlorine, or iodine). When photographic film, which consists of an emulsion (a thin layer of gelatin) and a base of transparent cellulose acetate or polyester, is exposed to light, silver-halide crystals suspended in the emulsion undergo chemical changes to form what is known as a latent image on the film. When the film is processed in a chemical agent called a developer, particles of metallic silver form in areas that were exposed to light. Intense exposure causes many particles to form, while weak exposure causes few to form. The image produced in this manner is called a negative because the tonal values of the subject photographed are reversed—that is, areas in the scene that were relatively dark appear light, and areas that were bright appear dark. The tonal values of the negative are reversed again in the photographic printing process or, when preparing color transparencies (slides), in a second development process.

Photography, then, is based on chemical and physical principles. The sensitivity of silver halides to light is the primary chemical principle. The physical principles are governed by the physics of light . The generic term light refers to the visible portion of a broad range of electromagnetic radiation, which includes radio waves, gamma rays, X rays, infrared, and ultraviolet rays. The human eye is sensitive to only a narrow band of electromagnetic wavelengths, called the visible spectrum. The spectrum comprises the full range of color tones. To the eye, the longest wavelengths register as red, the shortest as blue.

Photographic Film

Photographic films vary in the way they react to different wavelengths of visible light. Early black-and-white films were sensitive only to the shorter wavelengths of the visible spectrum—that is, to light perceived as blue. Later, colored dyes were added to film emulsions to make the silver halides responsive to light of other wavelengths. These dyes absorb light of their own color, making silver halide particles sensitive to light of that color. Orthochromatic film, incorporating yellow dyes in the emulsion and sensitive to all light but red, was the first improvement on simple blue-sensitive film.

In panchromatic film, the next major improvement, red-toned dyes were added to the emulsion, rendering the film sensitive to all visible wavelengths. Although slightly less sensitive to green tones than the orthochromatic type, panchromatic film is better able to reproduce the entire range of color tones. Thus, most films now used by amateur and professional photographers are panchromatic.

Two additional varieties of black-and-white film—process and chromogenic—have their special uses. Process film is used primarily for line reproduction of copy in the graphic arts. Such films have extremely high contrast, producing images with no tonal values between black and white. Chromogenic film produces dye images rather than silver images on the negative. Using dye couplers and silver halide in the emulsion, it can be developed by standard color-negative development processes. After development, the silver is bleached out of the film, leaving a black-and-white dye image.

Special-purpose films are sensitive to wavelengths beyond the visible spectrum. In addition to visible light, infrared film also responds to the invisible, infrared portion of the spectrum (see Infrared Photography, below).

Instant film, introduced by the Polaroid Corporation in the late 1940s, provides photographs within seconds or minutes of the taking of the picture, using a camera specially designed for this purpose. In instant film the processing chemicals and emulsion are combined in a self-contained envelope or on the print itself. Exposure, development, and printing all take place inside the camera. Polaroid, the leading manufacturer of this film, uses a conventional silver-halide emulsion. After the film is exposed and a negative image produced, the negative is sandwiched with photographic paper and processing chemicals, and a fogging agent transfers the negative image to the paper, producing a print. A number of instant films are manufactured in a 35-millimeter format, both in black and white and in color.

Color Film

Color films are more complex than black and white because they are designed to reproduce the full range of color tones as color, not as black, white, and gray tones. The design and composition of most color transparency films and color negative films are based on the principles of the subtractive color process, in which the three primary colors—yellow, magenta, and cyan (blue-green)—are combined with their complements to reproduce a full range of colors. Such films consist of three silver halide emulsions on a single layer. The top emulsion is sensitive only to blue. Beneath this is a yellow filter that blocks blues but transmits greens and reds to the second emulsion, which absorbs greens but not red. The bottom emulsion records reds.

When color film is exposed to light by a camera, latent black-and-white images are formed on each of the three emulsions. During processing, the chemical action of the developer creates actual images in metallic silver, just as in black-and-white processing. The developer combines with dye couplers incorporated into each of the emulsions to form cyan, magenta, and yellow images. Then the film is bleached, leaving a negative image in the primary colors. In color transparency film, unexposed silver-halide crystals not converted to metallic silver during the initial development are converted to positive images in dye and silver during a second stage of development. After the development action has been arrested, the film is bleached and the image fixed on it.

Film and Camera Formats

Different types of cameras require particular forms and sizes of film. Currently, the most widely used camera format is the 35 millimeter or small format, which produces 20, 24, or 36 images that each measure 24 by 36 millimeters on a roll of film. The film is wound on a spool inside a lighttight magazine or cartridge. Film for 35-millimeter cameras is also available in bulk, in long rolls that can be fed into individual cartridges and cut to length.

The next larger standard camera format, medium format, uses film sizes designated as either 120 or 220. Medium-format cameras produce images of various sizes, such as 6 by 6 centimeters or 2ј by 2ј inches, 6 by 7 centimeters, and 6 by 9 centimeters, depending on the configuration of the camera. Larger cameras, called view cameras, use sheet film. Standard sheet-film sizes correspond to standard view-camera formats: 4 by 5 inches, 5 by 7 inches, and 8 by 10 inches. Larger special-purpose view cameras, up to a 20-by-24 inch format, are in limited use.

Film Speed

Film is classified by speed as well as by format. Film speed is defined as an emulsion's degree of sensitivity to light, and it determines the amount of exposure required to photograph a subject under given lighting conditions. The manufacturer of the film assigns a standardized numerical rating in which high numbers correspond to “fast” emulsions and low numbers to “slow” ones. The standards set by the International Standards Organization (ISO) are used throughout the world, although some European manufacturers still use the German Industrial Standard, or Deutsche Industrie Norm (DIN). The ISO system evolved by combining the DIN system with the ASA (the industry standard previously used in the United States). The first number of an ISO rating, equivalent to an ASA rating, represents an arithmetic measure of film speed, whereas the second number, equivalent to a DIN rating, represents a logarithmic measure.

Low-speed films generally are rated from ISO 25/15 to ISO 100/21, but even slower films exist. Kodak's Rapid Process Copy Film, a special process film, has an ISO rating of 0.06/-12. Films in the ISO 125/22 to 200/24 range are considered medium speed, while films above ISO 200/24 are considered fast. In recent years, many major manufacturers have introduced superfast films with ISO ratings higher than 400/27. And certain films can be pushed well beyond their ratings by exposing them as though they had a higher rating and developing them for a greater length of time to compensate for the underexposure.

DX Coding is a recent innovation in film and camera technology. DX-coded cartridges of 35-millimeter film have printed on them a characteristic panel corresponding to an electronic code that tells the camera the ISO rating of the film as well as the number of frames on the roll. Many of the newer electronic cameras are equipped with DX sensors that electronically sense this information and automatically adjust exposures accordingly.

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