.. st placed in a type of projector called an enlarger. Light from the enlarger passes through the negative to a lens, which projects an enlarged or reduced image of the negative onto sensitized printing material. The process allows the photographer to reduce or increase the amount of light falling on particular portions of the printing material. Known as dodging and burning, these techniques render the final print lighter or darker in selected areas.
The printing material used in this process is a type of photographic paper similar in composition to that used for film, but much less sensitive to light. After it has been exposed, the print is developed and fixed by a process very similar to that used for developing film. In the finished print, areas exposed to the most light reproduce as dark tones, areas that were blocked from light by the negative reproduce as light tones, and areas exposed to moderate amounts of light reproduce as intermediate tones. Color prints from color negatives are made either by the projection method or by contact printing. Prints from color transparencies can be made directly by projection using the Cibachrome process or a Type R process, such as Kodak’s R-3 or Fuji’s Type 34. Alternatively, color transparencies can be printed by first making an intermediate negative, or internegative, which can then be printed either by contact or by projection.
A third color- printing process, called dye-transfer, is considerably more complex and is generally used only for professional work. Positive color transparencies and color negatives are printed on papers with multilayer emulsions containing color-forming agents. Examples of these are Fujichrome Type 34 process paper and Kodak Ektachrome, which are used for printing from color transparencies; and Ektacolor, Fujicolor, and Agfacolor CN Type A, which are used for printing from negatives. These papers are developed in dye-forming solutions without reversal processing. When color prints of this type are made, errors in exposure can be minimized by varying print exposure time.
Color balance is controlled by adjustable filters in the head of the enlarger, between the light source and the negative. In the dye-transfer process of making color prints, a separate negative is prepared for each of three colors: red, green, and blue. These color-separation negatives are either produced directly from the subject in a one-shot camera, now a relatively obsolete technique, or are produced indirectly from the color transparency. The negatives are then used to produce positive-relief images on gelatin sheets known as matrices. Three positive matrices are produced; one is steeped in red dye, another in blue, and the third in green.
After immersion, each matrix is printed in turn on a special easel that ensures precise alignment, or registration, to form a full-color image. Recent Technological Advances New technologies are beginning to blur the lines between photography and other image-making systems. In some new forms of still photography, silver-halide emulsions have been replaced by electronic methods of recording visual information. The Sony Corporation has developed a still-video camera called the Mavica, based on an earlier industrial model, the ProMavica. Unlike the conventional video camera, which uses magnetic tape, the Mavica records visual datalight reflected from objects in the scene photographedon a floppy disk.
The images are viewed on a monitor connected to the Mavica’s playback unit. Canon U.S.A. has also entered the still-video-camera market. Its RC-470 camera requires a still-video player for viewing, but the Xap Shot, which records 50 still images, with 300 to 400 lines of resolution, on a 5-cm (2-in) floppy disk, does not require any special equipment. It can be connected directly to a television receiver.
Paper prints of the recorded images can also be made, using a special, laser-driver computer printer. Digitization of photographic images has begun to revolutionize professional photography, giving rise to a specialized field known as image processing. Digitization of the visual data in a photographthat is, conversion of the data into binary numbers using a computermakes it possible to manipulate the photographic image by means of specially developed computer programs. The Scitex image-processing system, the commercial and advertising industry standard in the late 1980s, enables the operator to move or erase elements in a photograph, to change colors, to fashion composite images from several photographs, and to adjust contrast or sharpness. Other less sophisticated systems, such as Macintosh’s Digital Darkroom, allow similar operations. The quality of computer-generated images was, until recently, inferior to strictly photographic images.
Most nonindustrial color printers and laser printers cannot yet produce images with the tonal range, resolution, and saturation of photographs. Some systems, however, such as Presentation Technologies’ Montage Slidewriter and the Linotronic system, are capable of producing magazine-quality images. Special Techniques By the end of the 19th century, photography was already playing an important specialized role in astronomy. Since that time, many special photographic techniques have been developed. They serve as important tools in a number of scientific and technological areas.
High-Speed Photography and Cinematography Most modern cameras allow exposures with shutter speeds of up to 1/1000 second. Shorter exposure times can be attained by illuminating the object with a short light flash. In 1931 American engineer Harold E. Edgerton developed an electronic strobe light with which he produced flashes of 1/500,000 second, enabling him to photograph a bullet in flight. By the use of a series of flashes, the progressive stages of objects in motion, such as a flying bird, can be recorded on the same piece of film. Synchronization of the flash and the moving object is achieved by using a photocell to trigger the strobe light. The photocell is set up so that it is illuminated by a beam of light that is interrupted by the fast- moving object as soon as the object comes into the field of the camera. More recently, high-speed electro-optical and magneto-optical shutters have been developed that allow exposure times of up to a few billionths of a second.
Both types of shutters make use of the fact that the polarization plane of polarized light in certain materials is rotated under the influence of an electric or magnetic field. The magneto- optical shutter is made up of a glass cylinder that is placed inside a coil. A polarization filter is placed at each side of the glass cylinder. Both filters are crossed, and light that passes through the first filter becomes polarized and is stopped by the second filter. If a short electric pulse is passed through the coil, the polarization plane of the light in the glass cylinder is rotated, and light can pass through the system.
The electro-optical shutter, built in a similar way, consists of a cell with two electrodes that is filled with nitrobenzene and is placed between the two crossed polarization filters. The polarization plane inside the liquid is rotated by a short electrical pulse at the electrodes. Electro-optical shutters have been used to photograph the sequence of events during an explosion of an atomic bomb. Extremely fast motion can also be studied by high-speed cinematography. Conventional techniques, in which individual still photographs are taken in a fast sequence, allow a maximum rate of 500 frames per second.
By keeping the film stationary and using a fast rotating mirror (up to 5000 revolutions per second) that moves the images in a sequential order over the film, rates of a million pictures per second can be attained. For extremely high rates, such as a billion pictures per second, classical optical methods are abandoned and cathode ray tubes are used to make the exposures. Historical Development The term camera, as well as the apparatus itself, derives from camera obscura, which is Latin for dark room or dark chamber. The original camera obscura was a darkened room with a minute hole in one wall. Light entering the room through this hole projected an image from the outside on the opposite, darkened wall. Although the image formed this way was inverted and blurry, artists used this device, long before film was invented, to sketch by hand scenes projected by the camera.
Over the course of three centuries, the camera obscura evolved into a handheld box, and the pinhole was fitted with an optical lens to sharpen the image. 18th Century The photosensitivity of certain silver compounds, particularly silver nitrate and silver chloride, had been known for some time before British scientists Thomas Wedgwood and Sir Humphry Davy began experiments late in the 18th century in the recording of photographic images. Using paper coated with silver chloride, they succeeded in producing images of paintings, silhouettes of leaves, and human profiles. These photographs were not permanent, however, because the entire surface of the paper blackened after exposure to light. 19th Century The earliest photographs on record, known as heliographs, were made in 1827 by French physicist Joseph Nicphore Nipce. About 1831 French painter Louis Jacques Mand Daguerre made photographs on silver plates coated with a light-sensitive layer of silver iodide.
After exposing the plate for several minutes, Daguerre used mercury vapors to develop a positive photographic image. These photographs were not permanent because the plates gradually darkened, obliterating the image. In the first permanent photographs made by Daguerre, the developed plate was coated with a strong solution of ordinary table salt. This fixing process, originated by British inventor William Henry Fox Talbot, rendered the unexposed silver-iodide particles insensitive to light and prevented total blackening of the plate. The Daguerre method produced an unreproducible image on the silver plate for each exposure made.
While Daguerre perfected his process, Talbot developed a photographic method involving the use of a paper negative from which an unlimited number of prints could be made. Talbot had discovered that paper coated with silver iodide could be made more sensitive to light if dampened before exposure by a solution of silver nitrate and gallic acid, and that the solution also could be used in developing the paper after exposure. After development, the negative image was made permanent by immersion in sodium thiosulfate, or hypo. Talbot’s method, called the calotype process, required exposures of about 30 seconds to produce an adequate image on the negative. Both Daguerre and Talbot announced their processes in 1839.
Within three years the exposure time in both processes had been reduced to several seconds. In the calotype process, the grain structure of the paper negatives appeared in the finished print. In 1847 French physicist Claude Flix Abel Nipce de Saint-Victor devised a method of using a glass-plate negative. The plate, which was coated with potassium bromide suspended in albumin, was prepared before exposure by immersion in a silver- nitrate solution. The glass-plate negatives provided excellent image definition but required long exposures.
In 1851 British sculptor and photographer Frederick Scott Archer introduced wet glass plates using collodion, rather than albumin, as the coating material in which light- sensitive compounds were suspended. Because these negatives had to be exposed and developed while wet, photographers needed a darkroom close at hand in order to prepare the plates before exposure and to develop them immediately after exposure. Using wet collodion negatives and horse-drawn mobile darkrooms, photographers on the staff of American photographer Mathew B. Brady took thousands of photographs on battlefield sites during the American Civil War (1861-1865). Because use of the wet collodion process was limited largely to professional photography, various experimenters attempted to perfect a type of negative that could be exposed when dry and that would not require immediate development after exposure.
Advances were made by British merchant Richard Kennett, who supplied dry-plate negatives to photographers as early as 1874. In 1878 British photographer Charles Bennett produced a dry plate coated with an emulsion of gelatin and silver bromide, which was similar to modern plates. While experiments were being performed to increase the efficiency of black-and- white photography, preliminary efforts were made to use the coated-plate emulsions to produce natural color images of photographic subjects. In 1861 the first successful color photograph was made by British physicist James Clerk Maxwell, who used an additive- color process. About 1883, American inventor George Eastman produced a film consisting of a long paper strip coated with a sensitive emulsion.
In 1889 Eastman produced the first transparent, flexible film support, in the form of ribbons of cellulose nitrate. The invention of roll film marked the end of the early photographic era and the beginning of a period during which thousands of amateur photographers became interested in the new process. 20th Century In the early 20th century, commercial photography grew rapidly, and improvements in black-and-white photography opened the field to individuals lacking the time and skill to master the earlier, more complicated processes. The first commercial color-film materials, coated glass plates called Autochromes Lumireafter the process developed by French inventors Auguste and Louis Lumirebecame available in 1907. During this period, color photographs were produced with the three-exposure camera.
In the 1920s improvement of photomechanical processes used in printing created a great demand for photographs to illustrate text in newspapers and magazines. The demand for photographic illustrations with printed material established the new commercial fields of advertising and publicity photography. Technological advances, which simplified photographic materials and apparatus, encouraged the widespread adoption of photography as a hobby or avocation by great numbers of people. The 35-millimeter camera, which used small-sized film designed initially for motion pictures, was introduced in 1925 in Germany, and because of its compactness and economy, it became popular with both amateur and professional photographers. During this period, finely powdered magnesium was used by professional photographers as an artificial illuminant. Sprinkled in a trough and fired with a percussion cap, it produced a brilliant flash of light and a cloud of acrid smoke.
In the 1930s the photographic flashbulb replaced magnesium powder as a light source.