equipment, techniques, and processes used in the production of photographs. Figure 1: Sequence of negative-positive process, from the photographing of the original scene 1/4 The most widely used photographic process is the black-and-white negative-positive system (Figure 1). In the camera the lens projects an image of the scene being photographed onto a film coated with light-sensitive silver salts, such as silver bromide. A shutter built into the lens admits light reflected from the scene for a given time to produce an invisible but developable image in the sensitized layer, thus exposing the film. During development (in a darkroom) the silver salt crystals that have been struck by the light are converted into metallic silver, forming a visible deposit or density. The more light that reaches a given area of the film, the more silver salt is rendered developable and the denser the silver deposit that is formed there. An image of various brightness levels thus yields a picture in which these brightnesses are tonally reversed-a negative. Bright subject details record as dark or dense areas in the developed film; dark parts of the subject record as areas of low density; i.e., they have little silver. After development the film is treated with a fixing bath that dissolves away all undeveloped silver salt and so prevents subsequent darkening of such unexposed areas. Finally, a wash removes all soluble salts from the film emulsion, leaving a permanent negative silver image within the gelatin layer. A positive picture is obtained by repeating this process. The usual procedure is enlargement: the negative is projected onto a sensitive paper carrying a silver halide emulsion similar to that used for the film. Exposure by the enlarger light source again yields a latent image of the negative. After a development and processing sequence the paper then bears a positive silver image. In contact printing the negative film and the paper are placed face to face in intimate contact and exposed by diffused light shining through the negative. The dense (black) portions of the negative image result in little exposure of the paper and, so, yield light image areas; thin portions of the negative let through more light and yield dark areas in the print, thus re-creating the light values of the original scene. Additional reading General works include The Focal Encyclopedia of Photography, rev. ed., 2 vol. (1965, reissued 1977); L.P. Clerc, Photography: Theory and Practice, rev. and enlarged ed. edited by D.A. Spencer, 4 vol. (1970-71; originally published in French, 1926), a classic treatise; The Theory of the Photographic Process, 4th ed. edited by T.H. James (1977), a classic work; C.B. Neblette, Neblette's Handbook of Photography and Reprography: Materials, Processes, and Systems, 7th ed. edited by John M. Sturge (1977); Encyclopedia of Practical Photography, 14 vol. (1977-79), edited by and published for the Eastman Kodak Company; John Hedgecoe, The Photographer's Handbook: A Complete Reference Manual of Techniques, Procedures, Equipment, and Styles, 2nd ed. rev. (1982); and Bruce Pinkard, The Photographer's Bible: An Encyclopedic Reference Manual (1983). See also Albert Boni (ed.), Photographic Literature (1962), and a supplemental volume, Photographic Literature, 1960-1970 (1972), an exhaustive and valuable bibliography; Wolfgang Baier, Quellendarstellungen zur Geschichte der Fotografie: A Source Book of Photographic History (1963, reissued 1977), detailed bibliographies and references to the literature on photographic developments, with an introduction in English; and Beaumont Newhall, Latent Image: The Discovery of Photography (1967, reissued 1983), helpful for the understanding of the establishment of the medium.The evolution of photographic techniques is traced in H. Fox Talbot (William Henry Fox Talbot), The Pencil of Nature (1844-46, reprinted 1969), the inventor's account, illustrated with 24 actual calotypes; Georges Potonnie, The History of the Discovery of Photography (1936, reissued 1973; originally published in French, 1925), a detailed account of the early days of photography; Josef Maria Eder, History of Photography (1945, reprinted 1978; originally published in German, 4th rev. ed., 2 vol., 1932), a pioneer Austrian work that deals primarily with the scientific and technological development of photography; Beaumont Newhall (ed.), On Photography: A Sourcebook of Photo History in Facsimile (1956), an anthology of the inventors' own accounts of various processes; D.B. Thomas, The First Negatives: An Account of the Discovery and Early Use of the Negative-Positive Photographic Process (1964); Joseph S. Friedman, The History of Color Photography, 2nd ed. (1968); Helmut Gernsheim, The History of Photography from the Camera Obscura to the Beginning of the Modern Era, 2nd ed. (1969), the first part of which was revised as The Origins of Photography (1982); and Gail Buckland, Fox Talbot and the Invention of Photography (1980).Camera history and technology is outlined in Leslie D. Stroebel, View Camera Technique, 5th ed. (1986), on the use of studio and field cameras in industrial, commercial, and other applications; Michel Auer, The Illustrated History of the Camera from 1839 to the Present, trans. from French and adapted by D.B. Tubbs (1975); Brian Coe, Cameras: From Daguerreotypes to Instant Pictures (1978); and Eaton S. Lothrop, Jr., A Century of Cameras from the Collection of the International Museum of Photography at George Eastman House, rev. and expanded ed. (1982).Lenses and optical principles are described in C.B. Neblette and Allen E. Murray, Photographic Lenses, rev. ed. (1973); Arthur Cox, Photographic Optics, 15th rev. ed. (1974), classic manual of lens principles and use; and Sidney F. Ray, The Photographic Lens (1979), an introduction.Film and the techniques of taking pictures are examined in Walter Nurnberg, Lighting for Photography: Means and Methods, 16th rev. ed. (1968, reissued 1971); and Michael Langford, Basic Photography, 5th ed. (1986), and Advanced Photography: A Grammar of Techniques, 4th ed. (1980), manuals of practical technique for professional photographers.Film processing and printing are the subject of D.H.O. John and G.T.J. Field, A Textbook of Photographic Chemistry (1963), basics of chemical reactions in black-and-white processing; C.I. Jacobson and R.E. Jacobson, Developing: The Negative-Technique, 18th ed. (1972), manual of all aspects of negative technique; C.I. Jacobson and L.A. Mannheim, Enlarging, 22nd ed. (1975), manual of positive technique in black and white and colour; L.F.A. Mason, Photographic Processing Chemistry, 2nd ed. (1975), detailed treatment of processing mechanisms and reactions; Grant Haist, Modern Photographic Processing, 2 vol. (1979), chemistry and technology of black-and-white and colour processing; and Jan Arnow, Handbook of Alternative Photographic Processes (1982).Colour photography is treated in Louis Walton Sipley, A Half Century of Color (1951); Ralph M. Evans, W.T. Hanson, Jr., and W. Lyle Brewer, Principles of Color Photography (1953), principles of colour rendering, response, and reproduction; D.A. Spencer, Colour Photography in Practice, rev. ed. by L.A. Mannheim and Viscount Hanworth (1966, reissued 1975), containing both theory and practical techniques; R.W.G. Hunt, The Reproduction of Colour, 3rd ed. (1975), a standard handbook on colour photography, television, and printing, with moderately advanced mathematical treatment; and Gert Koshofer, Farbfotographie, 3 vol. (1981), a complete historical review, including a lexicon of equipment and materials.Special photographic techniques and applications are the focus of Harold E. Edgerton and James R. Killian, Jr., Flash! Seeing the Unseen by Ultra High-Speed Photography, 2nd ed. (1954), and Moments of Vision: The Stroboscopic Revolution in Photography (1979, reprinted 1984); J. Bergner, E. Gelbke, and W. Mehliss, Practical Photomicrography (1966; originally published in German, 1961), a comprehensive manual; R.F. Saxe, High-Speed Photography (1966), a condensed but comprehensive survey; John Brackett Hersey (ed.), Deep-Sea Photography (1967); C.R. Arnold, P.J. Rolls, and J.C.J. Stewart, Applied Photography (1971), on scientific applications; H. Lou Gibson, Photography by Infrared, 3rd ed. (1978); and Gjon Mili, Gjon Mili: Photographs and Recollections (1980), on stroboscopic photography. Helmut Erich Robert Gernsheim L. Andrew Mannheim Black-and-white processing and printing Negative development Amateurs usually process films in developing tanks. In this type of development roll or miniature film is wound around a reel with a spiral groove, which keeps adjacent turns separated and allows access by the processing solutions. Once the tank is loaded (in the dark), processing takes place in normal light, the processing baths (developer, intermediate rinse, fixer) being poured into the tank at the appropriate intervals. Sheet films are similarly treated in small tanks or held in hangers and immersed sequentially in the different processing solutions. Large-scale commercial processing laboratories use machines that automatically feed the films through the solutions in proper sequence. Developers and their characteristics The developer consists typically of one or more developing agents, a preservative (such as sodium sulfite) to prevent oxidation by the air, an alkali (such as sodium carbonate) to activate the developer, and a restrainer or antifoggant to ensure that the developer acts only on exposed silver halide crystals. A developer's main characteristics are activity, development speed, and effect on film gradation, graininess, and sharpness. Developers may be prepared on the basis of published formulas or bought as ready-mixed powders or concentrates for dilution with water. The developer is allowed to act for a specific time to build up the image to the required density and contrast. This time depends on the developer, the temperature, the degree of agitation, and the film-as indicated by recommendations from film and developer manufacturers. Black-and-white films The latent image The sensitive surface of ordinary film is a layer of gelatin carrying minute suspended silver halide crystals or grains (the emulsion)-typically silver bromide with some silver iodide. Exposure to light in a camera produces an invisible change yielding a latent image, distinguishable from unexposed silver halide only by its ability to be reduced to metallic silver by certain developing agents. Current theories postulate that silver halide crystals carry minute specks of metallic silver-so-called sensitivity specks-which amount in mass to about 1/100,000,000 part of the silver halide crystal. A silver halide is a compound of silver with fluorine, chlorine, bromine, or iodine, but only the last three are light-sensitive. When light action releases electrons from the silver halide crystal, they migrate to the sensitivity specks. The resulting electric charge on the specks attracts silver ions from the neighbouring silver halide; and as the silver ions accumulate, they become metallic silver, causing the speck to grow. Halogen (e.g., bromine) atoms at the same time migrate to the surface of the silver halide crystal and are there absorbed by the gelatin of the emulsion. When the sensitivity speck is large enough, it provides a point of attack for the developer, which can then reduce the whole silver halide crystal to silver. Developers are selective organic reducing agents that attack only silver halide crystals that have sufficiently large sensitivity specks. The halide grains carrying a developable sensitivity speck make up the latent image. Sensitometry and speed The sensitivity or speed of a film determines how much light it needs to produce a given amount of silver on development. Sensitometry is the science of measuring this sensitivity, which is determined by giving the material a series of graduated exposures in an appropriate instrument (the sensitometer). After development under specified conditions, the density of the silver deposit produced by each exposure is measured and the densities are plotted on a graph against the logarithm of the exposure. The resulting characteristic curve, or D/log E curve (see below Contrast), shows how the film reacts to exposure changes. A specified point on the curve also serves as a criterion for calculating film speed by methods laid down in various national and international standards. The internationally adopted scale is ISO speed, written, for example, 200/24. The first half of this (200) is arithmetic with the value directly proportional to the sensitivity (and also identical with the still widely used ASA speed). The second half (24) is logarithmic, increasing by 3 for every doubling of the speed (and matching the DIN speeds still used in parts of Europe). A film of 200/24 ISO is twice as fast (and for a given subject requires half as much exposure) as a film of 100/21 ISO, or half as fast as a film of 400/27 ISO. All-around films for outdoor and some indoor photography have speeds between 80/20 and 200/24 ISO; fine-grain films for maximum image definition between 25/15 and 64/19 ISO; and high-speed and ultraspeed films for poor light from 400/27 ISO up. Other film characteristics Contrast Colour photography Colour reproduction Present-day colour photographic processes are tricolour systems, reproducing different colours that occur in nature by suitable combinations of three primary-coloured stimuli. Each of these primary colours-blue-violet, green, and red-covers roughly one-third of the visible spectrum. Tricolour impressions can be produced by combining coloured lights (additive synthesis) or by passing white light through combinations of complementary filters, each of which holds back one of the primary colours (subtractive synthesis). In additive synthesis a combination of red and blue-violet light (e.g., light beams of the two colours directed on the same spot of a white screen) gives a purplish pink (magenta); equal parts of red and green produce yellow, and equal parts of green and blue-violet produce bluish green (cyan). Superimposition of all three light beams on a screen yields white; combinations of varying proportions of two or three of the colours produce virtually all the other hues. In subtractive synthesis yellow, magenta, and cyan filters or dye layers subtract varying proportions of the primary colours from white light. The yellow filter absorbs the blue component of white light and so controls the amount of blue present in a white-light beam that has passed through the filter. Similarly, the magenta filter controls the amount of green light left, and the cyan controls the amount of the red component. A cyan and a magenta filter superimposed in a white-light beam hold back both the red and the green component, making the emerging beam blue. Similarly, a cyan and a yellow filter together yield green, and a yellow and a magenta filter together yield red. Superimposing such filters or dye images of different densities in a white-light beam can therefore re-create any colour impression in the same way as superimposing light beams of the primary colours. The difference between additive and subtractive synthesis is the approach: in additive synthesis colours are built up by combining different intensities of primary-coloured light, and in subtractive synthesis colours are achieved by removing different proportions of primary-coloured light from white light. Most modern colour films are based on subtractive synthesis. Either method of colour synthesis should be capable of reproducing every existing colour in nature. In practice, the reproduction is imperfect; no filter dyes meet the required ideal specifications. Nevertheless, for most purposes reproduction is adequate. Colour films Reversal (slide) films Figure 6: Colour reproduction sequence with subtractive reversal film (see text). To reproduce colour by subtractive three-colour synthesis (Figure 6), colour films first break down the colours of an image into their primary components by means of three separate sensitized layers, each of which responds exclusively to blue, green, or red light. The image in each layer is reversal-processed to yield a positive dye image in a colour complementary to the layer's spectral sensitivity. Thus, the blue-sensitive layer first yields a negative image of everything blue in the original scene (e.g., the blue sky) and then a positive image of everything that is not blue. This positive image is coloured yellow. Similarly, the green-recording layer yields a magenta positive image of everything that is not green, and the red-recording layer a positive cyan image of everything that is not red. Blue sky, for instance, does not figure in the yellow positive image but does figure in the magenta positive image (not being green) and in the cyan positive image (not being red). The magenta and cyan dyes in the areas that were blue sky are superimposed, and white light passing through the resulting transparency loses its green and red, but not its blue, component; thus, the sky appears blue. Similarly, green subject components end up as positive yellow image density in the blue-recording and positive cyan density in the red-recording layer, combining to green in the transparency. Yellow records as a negative image in the green-recording and red-recording layers, hence leaving a positive yellow image only in the blue-recording layer. All other colours are formed by similar combinations of different densities of the dye images. Instant-picture photography History and evolution Cameras with built-in processing facilities, to reduce the delay between exposure and the availability of the processed picture, were proposed from the 1850s onward. The ferrotype process later adapted for "while-you-wait" photography by itinerant street and beach photographers goes back almost as far. Because of the messiness of handling liquid chemicals in or just outside the camera, such systems remained largely impractical. In the 1940s Edwin H. Land, a U.S. scientist and inventor, designed a film configuration that included a sealed pod containing processing chemicals in a viscous jelly or paste form to permit virtually dry processing inside the camera and yield a positive print within a minute or less of exposure. Land demonstrated (1947), and through his Polaroid Corporation marketed (1948), a camera and materials that realized this system. It used a positive sheet and negative emulsion, the latter being discarded after use. An instant-print colour film (Polacolor) was introduced in 1963 and an integral single-sheet colour film in 1972. After the mid-1970s other manufacturers offered similar instant-print processes. In 1977 Polaroid introduced an 8-mm colour movie film, and in 1982 it introduced still transparency films that permit rapid processing outside the camera. Black-and-white diffusion transfer The Polaroid process is based on negative paper carrying a silver halide emulsion and a nonsensitized, positive sheet containing development nuclei. After the exposure the two sheets are brought into intimate contact by being pulled between a pair of pressure rollers. These rupture a sealed pod (attached to the positive sheet) to spread processing chemicals-in the form of a viscous jelly-between the two sheets. This reagent develops a negative image and causes the silver salts from the unexposed areas to diffuse into the positive layer and deposit metallic silver on the development nuclei. After about 30 seconds to one minute the negative and positive sheets are peeled apart and the negative can be discarded. In special versions of the process the negative may be washed and treated to give a conventional negative for normal enlarging. In the original Polaroid instant-picture process the material was a dual roll of negative and positive sheets. Later versions of this peel-apart process use film packs and sheet films. They require special cameras incorporating the pressure rollers thatoperate the spread of processing jelly while the peel-apart sandwich is fed out of the camera. Special camera backs with this mechanism allow the use of Polaroid materials in professional cameras taking interchangeable film holders or magazines. Peel-apart Polaroid systems include high-speed emulsions, high-contrast, process, transparency, and scientific materials. Silver diffusion-transfer processes were invented in 1939 in Belgium and Germany and were used for a number of years in office copying systems until superseded by dry copying processes. Picture-taking technique The main areas of practical camera handling in photography concern sharpness control, exposure, and lighting. Sharpness control The image on the film is sharpest when the lens is focused to the exact object distance. Usually, however, a scene includes objects at varying distances from the camera. Various factors affect the sharpness distribution in a picture of such a scene. Special photosensitive systems The high working speed (efficiency of converting light into permanent images) of silver halides makes them almost the only materials suitable for camera use. Numerous light-sensitive systems not using silver have been known since the beginning of photography. In view of silver's high price, a number of substitute systems have grown in importance, and new ones have appeared. Most of them are limited to office copying, microfilming, the graphic arts, and other applications in which flat copy is reproduced. Electrophotography Electrophotography covers a number of processes that rely on photoconductive substances whose electrical resistance decreases when light falls on them. A layer of such a substance with a grounded backing plate is given a uniform electrostatic charge in the dark. When a light image is projected onto the surface, the photoconductor allows the electrostatic charge to leak away in proportion to the exposure. This leaves an "image" charge that can be converted, in various ways, into a visible image. In xerography the photoconductive layer is selenium, and the image is made visible by dusting the plate with an electrostatically charged powder (toner) having a charge that is the opposite of that of the electrostatic image. The powder adheres to the image portions only and is then transferred to a sheet of plain paper also under the influence of electrostatic fields. A final heat treatment fuses the powder into the paper for a permanent picture. The process usually makes a positive from a positive original. In office copying machines (the main application of xerography) the whole operating sequence is programmed and automated. A zinc oxide-coated paper may replace the selenium plate; if so, the pigment powder deposit is fused directly into the paper surface. The process is used mainly for line images without intermediate tones between black and white. Modified procedures permit continuous-tone reproduction and-with coloured pigments-also colour printing. In the electroplastic process a transparent thermoplastic serves as the photoconductive layer. After the plastic is charged and exposed, the residual electrostatic charge forms stresses in the thermoplastic. Controlled heating deforms the surface in the image areas into a grain pattern, which is frozen into the plastic on cooling. The resulting image is light-scattering and is viewed by reflection or in special projection systems. The photography industry Present-day manufacture of cameras and other photographic equipment is concentrated in mass-production plants that make most of the components (camera bodies, lenses, shutters, and other parts) on largely automated machines; the components are then assembled by semiskilled or skilled labour. Smaller manufacturers of low- and medium-priced cameras obtain components for assembly from such specialist suppliers as shutter manufacturers and lens producers. High-quality precision cameras are produced on a smaller scale with automated fabrication of the engineering components but much more extensive manual assembly by highly skilled technicians. Components and functions of every camera are tested at every production stage; less expensive cameras are usually batch-tested by a sampling procedure. The raw material for lens manufacture covers a range of optical glasses of different optical characteristics. About 10 major worldwide glass producers supply the several dozen optical firms offering lenses of well-known brands. The glass is cast into blanks for specific lens elements and ground and polished to the required exact specifications, with the elements assembled in metal (sometimes plastic) mounts. Extensive production tests and optical performance checks safeguard quality standards. Film base is produced either by coating a solution of the base material on large drums, where it solidifies (film casting), or by extrusion of plastics, such as polyester, in film extruders. For print materials, paper of suitable purity is coated with a barium sulfate emulsion in gelatin, to provide a smooth white surface, and then with the silver halide emulsion. Silver halide emulsions are made by mixing silver nitrate with a solution of alkali halide-typically potassium bromide and iodide-in gelatin. The silver halide then precipitates out as fine crystals. After cooling to a jelly, shredding, and washing, the emulsion is remelted and treated to increase speed and contrast. Colour sensitizers (and colour couplers for colour emulsions) and additives are introduced, and the gelatin emulsion is machine-coated on wide continuous webs of paper or film. Generally several coatings are applied-up to a dozen for certain colour films. Operations from emulsion mixing onward are carried on in total darkness. After cooling and drying, the material is batch-tested for consistent characteristics and then is cut and packed. Photofinishing laboratories process most amateur and some professional photographers' films and prints. In the 1980s, virtually all of the total business of the laboratories in the United States was in colour processing. Photofinishing laboratories use machines that carry the films in spliced-together lengths or on racks through successive tanks of the processing solutions. Prints are usually made to standard formats on automatic enlargers, taking both the negatives and the paper in continuous rolls. The paper rolls of 250 or 500 feet are processed in continuous-strip processors, which deliver prints dry and ready for automatic cutting. Many printers have automatic exposure measurement based on overall negative density, with automatically controlled colour correction for colour negatives. High-capacity colour printers of this type can produce 2,000 to 3,000 prints per hour. Coding systems identify individual films and corresponding prints by customer or order number for final re-sorting. More exacting processing services grade colour negatives before printing by light transmission measurements through different colour filters; the resulting exposure data may be punched as edge codes in the film itself or programmed on perforated paper tape. When the tape is run through the printer together with the film, the perforations directly control the colour exposures and corrections. Advanced automatic printing systems may involve electronically controlled image enhancement. Enlargements to special sizes and colour printing for professional photographers require individual enlarging by skilled personnel on conventional enlargers with advanced automation features of focusing, exposure measurement, and colour control. Other processing services include duplication of transparencies, various types of photocopying (partly on coin-operated copiers in public places), microfilming, and microfilm processing. L. Andrew Mannheim The Editors of the Encyclopdia Britannica

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