The term photography (from the Greek, phos, "light," and graphein, "to draw") means the production of a negative or positive black-and-white or colored record initiated by the action of radiant energy--usually in the form of light--upon a sensitive surface. The fundamental physical principle of photography is that light falling briefly on the grains of certain insoluble silver salts (silver chloride, bromide, or iodide) produces small, invisible changes in the grains. When placed in certain chemical solutions known as developers, the affected grains are converted into a black form of silver. When a photograph is taken with a camera, light reflected from the object passes through the shutter, diaphragm, and lens to form a real inverted image. For the brief period during which the shutter is open, this image falls on the surface of a film or plate sensitized by silver salts and causes an invisible latent image to be recorded on it. With an instant camera (see Instant Cameras under \Tcamera\t), development and printing are carried out while the film is still in the camera. In most cameras, however, pictures are taken until all the film has been exposed; then the still-undeveloped film must be removed in darkness or in greatly subdued light and placed in a developing solution. This solution darkens the affected grains of silver salt and converts the latent image into a negative image, in which dark and light areas in the object are recorded as light and dark areas, respectively, on the negative. The negative is then placed in a fixing solution, which dissolves the unaffected grains of silver salt and prevents any further action by light on the image. To produce a positive contact print, light is passed through the negative so that it falls on a piece of printing paper held in close contact with the negative. Once again a latent image is produced; it is then developed into a positive image and fixed. Just as with the formation of the negative, this process reverses the dark and light areas and reproduces the original tones of the subject in a positive print. Negatives today are usually too small to make a useful contact print. An optical \Tenlarger\t is therefore used to throw a magnified image of the negative onto the printing paper, which is then developed and fixed to yield a large positive print called an enlargement. CAMERA A camera consists essentially of a box carrying a lens, diaphragm, and shutter that are arranged to throw an image of the scene to be recorded onto a sensitive film or plate. The \Tlens\t is usually made up of several components. It forms a real, inverted image of the object. In the popular 35-mm cameras the focal length is typically 50 mm (2 in), but it can be shorter or longer according to the size of the camera. In the focusing mechanism provision is made for moving the lens backward or forward to focus the image on the film. Three main methods are used to determine the position of the lens for correct focus: focusing scale, range finder, and reflex finder. Two types of shutters are commonly used. The between-the-lens shutter is mounted between the components of the lens. The focal-plane shutter consists of a roller blind containing a slit that moves rapidly across the plane in front of the film. In popular cameras the shutter provides a range of exposures from about 1 second to 1/1,000 of a second. The diaphragm may also be placed between the components of the lens. It provides a circular hole of variable size that regulates the amount of light that reaches the film. If the light is weak, or if a short exposure is required, the diaphragm is opened wide to admit sufficient light. Under good lighting conditions with moderate exposures the diaphragm is set to a smaller \Taperture\t, thus reducing the amount of light reaching the film. The smaller aperture can also reduce effects of aberrations and of any error in focusing, thus producing a sharper picture (see \Tdepth of field\t). Of the various kinds of viewfinders, the simplest consists of a small hole, which serves to position the eye, and a wire frame a few inches in front of the hole, which delimits the field of view that corresponds to the image on the film. Most cameras today use optical or through-the-lens viewfinders. Types of Cameras The wide range of camera types is largely determined by the degree of compactness and portability required. The range extends from the ultraminiature camera, which weighs a few ounces and has a picture size of the order of 1 cm (0.4 in) square, to the large studio or view cameras weighing many kilograms or pounds and taking a picture of 20 X 25 cm (8 X 10 in) or more in size. Specialized types of cameras are also used, for example, for aerial or underwater photography. In some cameras the film is advanced by a motor drive for taking a short, rapid succession of still photos. Others operate in conjunction with multiple flash to produce a number of superimposed pictures. See the article \Tcamera\t for a description of the new types of electronically controlled cameras. FILM Photographic film has a number of physical characteristics and techniques associated with it. See also the separate articles \Tfilm, photographic\t and \Tfilm processing\t. Composition Film for black-and-white photography consists of a transparent base on which is spread a thin layer of an emulsion consisting of a suspension of minute mixed crystals of silver bromide and silver iodide in gelatin. The gelatin not only holds the grains but also greatly increases their sensitivity to light. The formation of the latent image was explained above. This image is probably made up of minute specks of silver formed in the grains in response to light, which act as nuclei enabling the developer to convert the whole of the rest of a grain into silver. The visible spectrum extends from about 400 to 700 nm, and the crystals are sensitive only to wavelengths of 500 nm or less. (1 nanometer = 10 to the power of -9). Sensitizing dyes are added to the emulsion to extend the sensitivity to include the longwave (red) end of the spectrum. Grain and Speed Films vary in the size of the grains they contain. This variation, in turn, has an effect on the film's speed, or sensitivity to light. Coarse-grain films are faster than those of fine grain, probably because a larger grain intercepts more light. Faster films can therefore be used with a shorter exposure, in a weaker light, or at a smaller aperture. The resultant picture, however, is apt to show grain, especially when greatly enlarged. Slower, fine-grain films are used for work of highest quality. The speed of a film is measured using a number of systems; for example, the American Standards Association (ASA) scale is one common measure. Slow films have speeds of around ASA 50; ASA 200-400 is satisfactory for general purposes; ASA 1,500 is the speed of an extremely fast film. The scale is linear, so that films of speeds ASA 100 and 200 require one-half and one-quarter, respectively, of the exposure required for a film of ASA 50. Developing The exposed film is transferred in darkness into a developing solution, which causes any affected grain of the latent image to be wholly converted into silver. After a brief rinse in a stop bath of acid or water to remove developer and to stop development, the film is fixed in a solution of sodium thiosulfate (often called hypo) or in a more rapid fixing solution; these solutions dissolve and remove the unchanged silver salts, thus rendering the film insensitive to light. The film is then washed and dried. Each picture is now a transparent negative in which light parts of the object are represented by dark areas and dark parts by light areas. Printing A large negative can be used to make a contact print, but small negatives are usually placed in an enlarger in which light from a lamp throws an enlarged and accurately focused image onto a piece of printing paper that has been coated with an emulsion similar to that on the film but less sensitive to light. The paper is then developed and fixed to form a positive print. Contrast A print in which the black areas are extremely black and the white areas extremely white is said to show high contrast, while one in which only various shades of gray appear is said to be of low contrast. The degree of contrast depends on the scene itself, but it also depends on the enlarger, the film, the exposure, and the development time (longer development gives higher contrast). Different grades of paper, ranging from hard (contrasty) to soft (low contrast), are also used to control the degree of contrast in the final print. LIGHTING A photograph is no more than a visual record of the variation in the brightnesses of different parts of a scene. Because the ratio of intensity of illumination from bright sunlight to the dim lighting of a dark interior is of the order of one million to one, two fundamental requirements for good photography are a way to provide appropriate lighting and a way to measure how much light is available. Natural daylight from two hours after sunrise to two hours before sunset varies according to weather conditions by a factor of only about 10, corresponding to a varying aperture of slightly more than three stops of the diaphragm; under these conditions exposure may be set by following some simple rules based on the conditions: bright sunlight, cloudy bright, open shade, or cloudy dull. Exposure Meter Under more complicated lighting conditions, an exposure meter should be used (see \Tactinometer\t). One type of exposure meter consists of a \Tphotoelectric cell\t connected to an ammeter. Light falling on the cell causes a current to flow; the deflection of a needle or some other indication gives a reading of the intensity of the lighting. The meter can be separate from the camera or can form an integral part of it. A popular arrangement is to have the photocell mounted inside the camera so that it receives a part of the light forming the photographic image. Light can be directed onto the cell in one of several ways without seriously impairing the image itself. The method is known as TTL (through-the-lens) metering. In one TTL arrangement, called match-the-needle metering, the photographer adjusts the exposure time or the stop (diaphragm setting) or both to bring a needle visible in the viewfinder into alignment with a fixed mark. A more modern arrangement is for the photographer to preset one of the two settings (shutter speed or aperture); the other setting is then controlled automatically by the photocell. A TTL measurement method that depends on light reflected from the subject is excellent because it totally eliminates the effect of variations in the intensity of the incident light; in practice it is effective with average subjects. It does have its drawbacks, however. For example, suppose automatic aperture control is used to take three positive color transparencies: the first of a white, the second of a gray, and the third of a black piece of paper (black paper reflects about 10% of the incident light). The automatic aperture control would produce three images of identical intensity. The three pictures will show an identical shade of gray even though the three original subjects varied considerably in brightness. An alternative method, incident light measurement, eliminates the above drawback. The incident light meter is placed in the scene to be photographed and is usually pointed at the light source. The meter reading indicates the exposure (combination of shutter speed and f-stop) appropriate for a film of a given speed. Dark and light objects in the scene will receive identical exposures and will be appropriately recorded as dark or light in the picture. A TTL meter can achieve the same effect if it is directed at a gray card placed in the scene to be photographed; the reading will depend only on the incident light intensity, making the method an incident light method. Color Filters Filters placed over the lens of the camera are used to modify the light passing through the lens and onto the film. Because filters do not transmit all the light that reaches them, the exposure must usually be increased when a filter is used. A yellow filter absorbs blue rays and greatly improves many black-and-white pictures by darkening the sky. Its use requires an approximate doubling of the exposure. A red filter gives similar but stronger effects and may produce dramatic pictures of mountain scenery. Color filters may also be used to increase contrast. Suppose a dress has a pattern of orange and blue. A blue filter will allow blue rays to pass but will block orange rays, so that in the picture the orange areas will be darkened. A polaroid filter appropriately oriented will block the polarized light from the sky and is useful in darkening blue sky in color photographs; it can also be used to eliminate undesirable reflections in the picture. Lighting with Flash Bulbs When light is weak, a flash unit, mounted on or off the camera, can be used. The flash is produced by a flash bulb mounted in the center of a concave mirror and containing fine magnesium, aluminum, or zirconium wire in oxygen. Lighting with Electronic Flash The flash is produced by a capacitor charged to a high voltage discharging through a flash tube in as little as 1/10,000 sec, a time short enough to freeze rapid action. The firing of the flash is usually synchronized with the shutter action. A sensitive electronic eye, preferably mounted on the camera itself, may be used to operate a mechanism that extinguishes the flash when sufficient light for exposure of the film has been reflected back from the subject. Multiple flash may be used to produce a number of superimposed pictures; such a technique is useful for analyzing movement--in sports, for example (see \Tflash photography\t). COLOR PHOTOGRAPHY Like black-and-white photography, color photography depends fundamentally upon the darkening of silver salts. It is possible because any color can be made from mixtures of the three primary colors: red, green, and blue. (See \Tcolor\t; \Tcolor perception\t.) Each of these three primary colors has a complementary color, which when mixed with it gives white. Thus green and magenta are complementary colors. Two successive transformations into the complementary color restore the original color--for example, green, into magenta and magenta back into green. This effect forms the basis of the negative-positive process. The Negative-Positive Process The negative film consists of three superimposed layers: one blue-sensitive, one green-sensitive, and one red-sensitive. Exposure in the camera is followed by development, which produces deposits in black silver grains representing the blue, the green, and the red parts of the optical image. During development, substances known as dye intermediates incorporated in the three layers are converted into dyes that stain each of the three silver images in a color complementary to the color of the light that formed it. The silver itself is bleached and removed so that only the transparent color remains. In the negative, therefore, the blue parts of the picture are represented by yellow, the green parts by magenta, and the red parts by cyan. Next, the negative is placed in an enlarger, where the image is thrown onto printing paper coated with three superimposed layers of emulsion that are essentially the same as those in the negative color film; they are developed in essentially the same way. Just as the negative forms colors complementary to those in the optical image, so the paper forms colors complementary to those in the negative. The result is that the original colors of the subject are reproduced. The stages in the processing of the negative and print are quite a bit more elaborate than have been indicated in the simplified account given above; the time taken is about one hour, and stringent control of temperature of the solutions is required. In producing the color print, care is needed to ensure accurate color rendition. In one method, three successive exposures are made in the enlarger--one with blue, one with red, and one with green light--and the relative durations are adjusted from indications given by test strips. In an alternative method, a white light source is used with a single exposure. As successive test strips are made, color-correcting filters are placed over the lens of the enlarger and adjusted until a satisfactory test strip is obtained. The Reversal Method for Color Transparencies As explained above, red plus blue green equals white, green plus magenta equals white, and blue plus yellow equals white. Accordingly, blue green is the color produced from white light when red light is removed from it. Therefore blue green light can be thought of as minus red, magenta as minus green, and yellow as minus blue. Positive, or reversal, film consists of three superimposed emulsion layers sensitive to the red, green, and blue parts of the spectrum, respectively. As in the previous method, after exposure in the camera and a first development, three superimposed black-and-white images in silver are formed that represent the red, green, and blue parts of the optical image. In the reversal method, however, these negative images are not stained. The film is next exposed to light from a lamp, which induces the formation of a latent image in the previously unaffected grains of silver salt, that is, in those grains that were unaffected by the red, green, and blue lights in the optical image. A different dye intermediate is incorporated in each of the three layers. When the film is given the second development, a second set of three silver images is formed. The dye intermediates form dyes that stain the newly forming second images, but not the first ones, coloring them as follows: The red-sensitive layer produces the color cyan (minus red); the green-sensitive layer produces magenta (minus green); and the blue-sensitive layer produces yellow (minus blue). Both sets of silver images are bleached and washed away, so that only the transparent dyes remain. For example, in an area where the optical image was red, a black deposit was initially formed in the red-sensitive area; it is bleached and washed away, so that the layer is clear. In the green- and blue-sensitive layers the second silver images were stained minus green and minus blue, respectively, and only these colors remained after bleaching. When the transparency is viewed by projection of white light through it, the minus-green and minus-blue dyes remove the green and the blue components of the white light, leaving only the red. Thus the original red color in the optical image is correctly reproduced. Similar processes can be applied to the green and the blue parts of the optical image. Polaroid instant color film is developed through a process called diffusion transfer. The film itself is composed of 18 discrete, microscopically thin layers. As the exposed film emerges from the camera, a chemical developing agent is forced between the top layers, which hold the color dyes, and the bottom layers, on which the image is imprinted. The image develops below, while the chemicals producing the colors diffuse through the upper layers. Harry Asher Bibliography: Adams, Ansel, The Camera (1980); Burchfield, Jerry, Darkroom Art (1981); Craven, George, How Photography Works (1986); Eastman Kodak Staff, The Complete Book of Photography (1986) and More Joy of Photography (1988); Eisenstaedt, Alfred, Eisenstaedt's Guide to Photography (1981); Feininger, Andreas, The Creative Photographer, rev. ed. (1974); Focal Encyclopedia of Photography, 2 vols., rev. ed. (1965; repr. 1977); Freeman, Michael, ed., The Image (1987); Koschatzky, Walter, The Art of Photography (1985); Lyons, Nathan, Photographers on Photography (1966); Upton, Barbara L. and John, Photography, 3d ed. (1985).