Реферат: Photography Essay Research Paper Photography is a
Photography Essay, Research Paper
Photography is a 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. See also Motion Picture.
The Camera and Its Accessories
Modern cameras operate on the basic principle of the camera obscura (see Historical Development, below). Light passing through a tiny hole, or aperture, into an otherwise lightproof box casts an image on the surface opposite the aperture. The addition of a lens sharpens the image, and film makes possible a fixed, reproducible image. The camera is the mechanism by which film can be exposed in a controlled manner. Although they differ in structural details, modern cameras consist of four basic components: body, shutter, diaphragm, and lens. Located in the body is a lightproof chamber in which film is held and exposed. Also in the body, located opposite the film and behind the lens, are the diaphragm and shutter. The lens, which is affixed to the front of the body, is actually a grouping of optical glass lenses. Housed in a metal ring or cylinder, it allows the photographer to focus an image on the film. The lens may be fixed in place or set in a movable mount. Objects located at various distances from the camera can be brought into sharp focus by adjusting the distance between the lens and the film.
The diaphragm, a circular aperture behind the lens, operates in conjunction with the shutter to admit light into the lighttight chamber. This opening may be fixed, as in many amateur cameras, or it may be adjustable. Adjustable diaphragms are composed of overlapping strips of metal or plastic that, when spread apart, form an opening of the same diameter as the lens; when meshed together, they form a small opening behind the center of the lens. The aperture openings correspond to numerical settings, called f-stops, on the camera or the lens.
The shutter, a spring-activated mechanical device, keeps light from entering the camera except during the interval of exposure. Most modern cameras have focal-plane or leaf shutters. Some older amateur cameras use a drop-blade shutter, consisting of a hinged piece that, when released, pulls across the diaphragm opening and exposes the film for about 1/30th of a second.
In the leaf shutter, at the moment of exposure, a cluster of meshed blades springs apart to uncover the full lens aperture and then springs shut. The focal-plane shutter consists of a black shade with a variable-size slit across its width. When released, the shade moves quickly across the film, exposing it progressively as the slit moves.
Most modern cameras also have some sort of viewing system or viewfinder to enable the photographer to see, through the lens of the camera, the scene being photographed. Single-lens reflex cameras all incorporate this design feature, and almost all general-use cameras have some form of focusing system as well as a film-advance mechanism.
Cameras come in a variety of configurations and sizes. The first cameras, ?pinhole? cameras, had no lens. The flow of light was controlled simply by blocking the pinhole. The first camera in general use, the box camera, consists of a wooden or plastic box with a simple lens and a drop-blade shutter at one end and a holder for roll film at the other. The box camera is equipped with a simple viewfinder that shows the extent of the picture area. Some models have, in addition, one or two diaphragm apertures and a simple focusing device.
The view camera, used primarily by professionals, is the camera closest in design to early cameras that is still in widespread use. Despite the unique capability of the view camera, however, other camera types, because of their greater versatility, are more commonly used by both amateurs and professionals. Chief among these are the single- lens reflex (SLR), twin-lens reflex (TLR), and rangefinder. Most SLR and rangefinder cameras use the 35-millimeter film format, while most TLR as well as some SLR and rangefinder cameras use medium-format film?that is, size 120 or 220.
View cameras are generally larger and heavier than medium- and small-format cameras and are most often used for studio, landscape, and architectural photography. These cameras use large-format films that produce either negatives or transparencies with far greater detail and sharpness than smaller format film. View cameras have a metal or wood base with a geared track on which two metal standards ride, one in front and one in back, connected by a bellows. The front standard contains the lens and shutter; the rear holds a framed ground-glass panel, in front of which the film holder is inserted. The body configuration of the view camera, unlike that of most general-purpose cameras, is adjustable. The front and rear standards can be shifted, tilted, raised, or swung, allowing the photographer excellent control of perspective and focus.
Rangefinder cameras have a viewfinder through which the photographer sees and frames the subject or scene. The viewfinder does not, however, show the scene through the lens but instead closely approximates what the lens would record. This situation, in which the point of view of the lens does not match that of the viewfinder, results in what is known as parallax. At longer distances, the effects of parallax are negligible. At short distances, however, they become more pronounced, making it difficult for the photographer to frame a scene or subject with certainty.
Reflex cameras, both the SLR and the TLR types, are equipped with mirrors that reflect in the viewfinder the scene to be photographed. The twin-lens reflex is box-shaped, with a viewfinder consisting of a horizontal ground-glass screen located at the top of the camera. Mounted vertically on the front panel of the camera are two lenses, one for taking photographs and the other for viewing. The lenses are coupled, so that focusing one automatically focuses the other. The image formed by the upper, or viewing, lens is reflected to the viewing screen by a fixed mirror mounted at a 45-degree angle. The photographer focuses the camera and adjusts the composition while looking at the screen. The image formed by the lower lens is focused on the film at the back of the camera. Like rangefinder cameras, TLRs are subject to parallax.
In the SLR type of reflex camera, a single lens is used for both viewing the scene and taking the photograph. A hinged mirror situated between the lens and the film reflects the image formed by the lens through a five-sided prism and onto a ground-glass screen on top of the camera. At the moment the shutter is opened, a spring automatically pulls the mirror out of the path between lens and film. Because of the prism, the image recorded on the film is almost exactly that which the camera lens ?sees,? without any parallax effects.
Most SLRs are precision instruments equipped with focal-plane shutters. Many have automatic exposure-control features and built-in light meters. Most modern SLRs have electronically triggered shutters. Apertures, too, may be electronically actuated or they may be adjusted manually. Increasingly, camera manufacturers produce SLRs with automatic focusing, an innovation originally reserved for amateur cameras. Minolta’s Maxxum series, Canon’s EOS series, and Nikon’s advanced professional camera, the F-4, all have autofocus capability and are completely electronic. Central processing units (CPUs) control the electronic functions in these cameras (see Microprocessor). Minolta’s Maxxum 7000i has software ?cards? that, when inserted in a slot on the side of the camera, expand the camera’s capabilities (see Computer).
Autofocus cameras use electronics and a CPU to sample automatically the distance between camera and subject and to determine the optimum exposure level. Most autofocus cameras bounce either an infrared light beam or ultrasonic (sonar) waves off the subject to determine distance and set the focus. Some cameras, including Canon’s EOS and Nikon’s SLRs, use passive autofocus systems. Instead of emitting waves or beams, these cameras automatically adjust the focus of the lens until sensors detect the area of maximum contrast in a rectangular target at the center of the focusing screen.
Of the three most widely used designs, the SLR is the most popular among both professionals and amateurs. Its greatest advantage is that the image seen through the viewfinder is virtually identical with that on which the lens is focused. In addition, the SLR is generally easy and fast to operate and comes with a greater variety of interchangeable lenses and accessories than the other two camera types.
The rangefinder camera, previously used by photojournalists because of its compact size and ease of operation (compared with the big, slow 4-by-5 inch press cameras used by an earlier generation) has largely been replaced by the SLR. Rangefinder cameras, however, have a simpler optical system with fewer moving parts and are thus inherently more sturdy than SLRs, in addition to being quieter and weighing less. For these reasons, some photographers, mainly professionals, continue to use them.
Compared with the other two designs, TLRs have a relatively slow focusing system. As with rangefinder cameras, fewer interchangeable lenses are available, yet the TLR remains popular. The camera produces larger negatives than most SLRs and rangefinders, an advantage when fine detail must be rendered in the final image. In recognition of this, some manufacturers?including Hasselblad, Mamiya, Bronica, and Rollei?have combined the convenience of the SLR with the medium-film format, further reducing the market for the TLR.
Some cameras are designed primarily for amateurs: They are simple to operate, and they produce photographs acceptable to the average snapshot photographer. Many ?point-and-shoot? amateur cameras now employ sophisticated technology, with features such as autofocus and exposure-control systems that simplify the process of taking pictures and almost guarantee good-quality photos.
The lens is as important a part of a camera as the body. Lenses are referred to in generic terms as wide-angle, normal, and telephoto. The three terms refer to the focal length of the lens, which is customarily measured in millimeters. Focal length is defined as the distance from the center of the lens to the image it forms when the lens is set at infinity. In practice, focal length affects the field of view, magnification, and depth of field of a lens.
Cameras used by professional photographers and serious amateurs are designed to accept all three lens types interchangeably. In 35-millimeter photography, lenses with focal lengths from 20 to 35 millimeters are considered wide-angle lenses. They provide greater depth of field and encompass a larger field (or angle) of view but provide relatively low magnification. Extreme wide-angle, or fish-eye, lenses provide fields of view of 180 degrees or more. A 6-millimeter fish-eye lens made by Nikon has a 220-degree field of view that produces a circular image on film, rather than the normal rectangular or square image.
Lenses with focal lengths from 45 to 55 millimeters are referred to as normal lenses because they produce an image that approximates the field of view of the human eye. Lenses with longer focal lengths, called telephoto lenses, constrict the field of view and decrease the depth of field while greatly magnifying the image. For a 35-millimeter camera, lenses with focal lengths of 85 millimeters or more are considered telephoto.
A fourth generic lens type, the zoom lens, is designed to have a variable focal length, which can be adjusted continuously between two fixed limits. Zoom lenses are especially useful in conjunction with single-lens reflex cameras, for which they allow continuous control of image scale.
Developing and Printing
The latent image on film becomes visible through the process called developing?the application of certain chemical solutions to transform the film into a negative. The process in which this negative is used to create a positive image is called printing, and the image is called a print. Film is developed by treating it with a weak reducing alkaline chemical called the developing solution, or developer. This solution reactivates the process begun by the action of light when the film was exposed. The effect is to reduce further the silver-halide crystals in which metallic silver had already formed, so that large grains of silver form around the minute particles that make up the latent image.
As large particles of silver begin forming, a visible image becomes apparent on the film. The thickness and density of silver deposited in each area depend on the amount of light received by the area during exposure. In order to arrest the action of the developer, the film is then bathed in a weakly acidic solution, which neutralizes the alkaline developer. After rinsing, the negative image is fixed: Residual silver-halide crystals are removed, and remaining metallic silver particles are stabilized. The chemical solution used for fixing, commonly referred to as hypo, or fixer, is usually sodium thiosulfate, although potassium or ammonium thiosulfate may also be used. Fixer remover, or hypo clearing agent, is then used to rinse any remaining fixer from the film. Film must be rinsed thoroughly in water, as residual fixer tends to destroy negatives with time. Finally, bathing the processed film in a washing aid promotes uniform drying and prevents formation of water spots and streaks.
Printing is done by either of two methods: contact or projection. The contact method is used when prints of exactly the same size as the negative are desired. They are made by placing the emulsion side of the negative in contact with the printing material and exposing the two together under a source of light.
In projection printing, the negative is first 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 data?light reflected from objects in the scene photographed?on 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 photograph?that is, conversion of the data into binary numbers using a computer?makes 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.
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.
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.
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.
The earliest photographs on record, known as heliographs, were made in 1827 by French physicist Joseph Nic?phore Ni?pce. 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 F?lix Abel Ni?pce 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.
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 Lumi?re?after the process developed by French inventors Auguste and Louis Lumi?re?became 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.