Photographic Techniques, various techniques of producing permanent images on sensitized surfaces by means of the photochemical action of light or other forms of radiant energy; or the more recent techniques of capturing images by electronic means.
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. The history of photographic techniques shows that 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. This development has been paralleled in the cinema by the changing techniques and technologies of cinematography.
Light is the essential ingredient in photography. Nearly all forms of photography are based on the light-sensitive properties of silver-halide crystals, chemical compounds of silver and halogens (bromine, chlorine, or iodine). When the photographic film, which consists of an emulsion (a thin layer of gelatin) and a base of transparent cellulose acetate or polyester, is exposed to light, silver-halide crystals suspended in the emulsion undergo chemical changes to form what is known as a latent image on the film. When the film is processed in a chemical agent called a developer, particles of metallic silver form in areas that were exposed to light. Intense exposure causes many particles to form; weak exposure, a few. The image produced in this manner is called a negative because the tonal values of the subject photographed are reversed; that is, areas in the scene that were relatively dark appear light, and areas that were bright appear dark. The tonal values of the negative are reversed again in the photographic printing process or, when preparing colour transparencies (slides), in a second development process.
Photography, then, is based on chemical and physical principles. The sensitivity of silver halides to light is the primary chemical principle. The physical principles are governed by optics, the physics of light. The generic term “light” refers to the visible portion of a broad range of electromagnetic radiation, which includes radio waves, gamma rays, X-rays, infrared, and ultraviolet rays. The human eye is sensitive to only a narrow band of electromagnetic wavelengths, the visible spectrum. This spectrum comprises the full range of colour tones. To the eye, the longest visible wavelengths register as red, the shortest as blue.
II PHOTOGRAPHIC FILM
Photographic films vary in the way they react to different wavelengths of visible light. Early black-and-white films were sensitive only to the shorter wavelengths of the visible spectrum, that is, to light perceived as blue. Later, coloured dyes were added to film emulsions to make the silver halides responsive to light of other wavelengths. These dyes absorb light of their own colour, making silver halide particles sensitive to light of that colour. The orthochromatic film, incorporating yellow dyes in the emulsion and sensitive to all light but red, was the first improvement on simple blue-sensitive film.
In panchromatic film, the next major improvement, red-toned dyes were added to the emulsion, rendering the film sensitive to all visible wavelengths. Although slightly less sensitive to green tones than the orthochromatic type, panchromatic film is better able to reproduce the entire range of colour tones. Thus, most films now used by amateur and professional photographers are panchromatic.
Two additional varieties of black-and-white film—process and chromagenic—have their special uses. Process film is used primarily for line reproduction of copy in the graphic arts. Such films have extremely high contrast, producing images with no tonal values between black and white. Chromogenic film produces dye images rather than silver images on the negative. Using dye couplers and silver halide in the emulsion, it can be developed by standard colour negative development processes. After development, the silver is bleached out of the film, leaving a black-and-white dye image.
Special-purpose films are sensitive to wavelengths beyond the visible spectrum. In addition to visible light, an infrared film also responds to the invisible, infrared portion of the spectrum (see Infrared Photography, below).
Instant film, introduced by the Polaroid Corporation in the late 1940s, provides photographs within seconds or minutes of the taking of the picture, using a camera specially designed for this purpose. In instant film, the processing chemicals and emulsion are combined in a self-contained envelope or on the print itself. Exposure, development, and printing all take place inside the camera. Polaroid, the leading manufacturer of this film, uses a conventional silver-halide emulsion. After the film is exposed and a negative image produced, the negative is sandwiched with photographic paper and processing chemicals, and a fogging agent transfers the negative image to the paper, producing a print. A number of instant films are manufactured in a 35-mm format, both in black-and-white and in colour.
A Colour Film
Colour films are more complex than black-and-white films because they are designed to reproduce the full range of colour tones as colour, not as black, white, and gray tones. The design and composition of most colour transparency films and colour negative films are based on the principles of the subtractive colour process, in which the three primary colours, yellow, magenta, and cyan (blue-green), are combined with their complements to reproduce a full range of colours. Such films consist of three silver halide emulsions on a single layer. The top emulsion is sensitive only to blue. Beneath this is a yellow filter that blocks blues but transmits greens and reds to the second emulsion, which absorbs greens but not red. The bottom emulsion records reds.
When colour film is exposed to light by a camera, latent black-and-white images are formed on each of the three emulsions. During processing, the chemical action of the developer creates actual images in metallic silver, just as in black-and-white processing. The developer combines with dye couplers incorporated into each of the emulsions to form cyan, magenta, and yellow images. Then the film is bleached, leaving a negative image in the primary colours. In colour transparency film, unexposed silver-halide crystals not converted to metallic silver during the initial development are converted to positive images in dye and silver during the second stage of development. After the development action has been arrested, the film is bleached and the image fixed on it.
B Film and Camera Formats
Different types of camera require particular forms and sizes of film. Currently, the most widely used camera format is the 35 mm or small format, which produces 20, 24, or 36 images that each measure 24 x 36 mm on a roll of film. The film is wound on a spool inside a light-proof magazine or cartridge. Film for 35-mm cameras is also available in bulk, in long rolls that can be fed into individual cartridges and cut to length.
The next larger standard camera format, medium format, uses film sizes designated as either 120 or 220. Medium-format cameras produce images of various sizes, such as 6 x 6 cm or 2 x 2 in, 6 x 7 cm, and 6 x 9 cm, depending on the configuration of the camera. Larger cameras, called view cameras, use sheet film. Standard sheet film sizes correspond to standard view camera formats: 4 x 5 in, 5 x 7 in, and 8 x 10 in. Larger special-purpose view cameras, up to 20 x 24 in, are in limited use.
C Film Speed
The film is classified by speed as well as by format. Film speed is defined as an emulsion’s degree of sensitivity to light and determines the amount of exposure required to photograph a subject under given lighting conditions. The manufacturer of the film assigns a standardized numerical rating in which high numbers correspond to “fast” emulsions and low numbers to “slow” ones. The standards set by the International Standards Organization (ISO) are used throughout the world, although some European manufacturers still use the German Industrial Standard, or Deutsche Industrie Norm (DIN). The ISO system evolved by combining the DIN system with the ASA (the industry standard previously used in the United States). The first number of an ISO rating, equivalent to an ASA rating, represents an arithmetic measure of film speed, whereas the second number, equivalent to a DIN rating, represents a logarithmic measure.
Low-speed films are generally rated from ISO 25/15 to ISO 100/21, but even slower films exist. Kodak’s Rapid Process Copy Film, a special process film, has an ISO rating of 0.06/-12. Films in the ISO 125/22 to 200/24 range are considered medium speed, while films above ISO 200/24 are considered fast. In recent years, many major manufacturers have introduced superfast films with ISO ratings higher than 400/27. And certain films can be pushed well beyond their ratings by exposing them as though they had a higher rating and developing them for a greater length of time to compensate for the underexposure.
DX coding is a recent innovation in film and camera technology. DX-coded cartridges of a 35-mm film have printed on them a characteristic panel corresponding to an electronic code that tells the camera the ISO rating of the film as well as the number of frames on the roll. Many of the newer electronic cameras are equipped with DX sensors that electronically sense this information and automatically adjust exposures accordingly.
Differences in sensitivity of a film emulsion to light depend on various chemical additives. For example, hypersensitizing compounds increase film speed without affecting the film’s colour sensitivity. The high-speed film can also be manufactured by increasing the concentration of large silver-halide crystals in the emulsion. In recent years, a generation of faster, more sensitive films has been created by altering the shape of crystals. Flatter silver-halide crystals offer greater surface area. Films incorporating such crystals, such as Kodak’s T-grain Kodacolour films, have a correspondingly greater sensitivity to light.
The grain structure of faster films is generally heavier than that of slower films. Grain structure may give rise to a mottled pattern on prints that have been greatly enlarged. Photographs taken with slower-speed film appear less grainy when enlarged. Because of the small size of their silver-halide grains, slow-speed films generally have a higher resolution—that is, they can render fine details with greater sharpness—and can produce a broader range of tones than fast films. When tonal range and sharpness of detail are not as important as capturing a moving subject without blurring, fast films are used.
D Exposure Range
Each type of film has a characteristic exposure range or latitude of exposure. Latitude is basically the margin of error in exposure within which film, when developed and printed, can render the actual colour and tonal values of the scene photographed.
The terms “overexposure” and “underexposure” are used to characterize deviations, whether intentional or not, from the optimum exposure. Film exposed to light longer than optimal will often be “blocked up” with silver in highlight areas, resulting in a loss of contrast and sharpness and an increase in graininess. Underexposure, on the other hand, produces negatives characterized as thin, a condition in which there are not enough silver crystals for accurate rendering of dark and shadowed areas.
With films that have a narrow latitude, an exposure adjusted for a shady area is likely to result in overexposure of adjacent sunny areas. The greater a film’s latitude, the greater its ability to provide satisfactory prints despite over- or underexposure. Films from which negatives are made, both colour and black-and-white, generally offer enough latitude to allow the photographer a certain margin of error. Transparency films, from which colour slides are made, generally have less latitude.
III THE CAMERA AND ITS ACCESSORIES
Modern cameras operate on the basic principle of the camera obscura. Light passing through a tiny hole, or aperture, into an otherwise light-proof box casts an image on the surface opposite the aperture. The addition of a lens sharpens the image, and the 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 light-proof chamber (Latin, camera) 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 attached 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 light-proof 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 centre 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 (SLRs) all incorporate this design feature, and almost all general-use cameras have some form of focusing system as well as a film-advance mechanism.
A Exposure Control
By adjusting shutter speed and diaphragm aperture, the photographer obtains just enough light to ensure a proper exposure. Shutter speed and aperture setting are directly proportional: a one-increment change in shutter speed is equal to a change of one f-stop. A “one-stop” adjustment in exposure can refer to a change in either shutter speed or aperture setting; the resulting change in the amount of light reaching the film will be the same. Thus, if the shutter speed is increased, a compensatory increase must be made in aperture size to allow the same amount of light to reach the film. Fast shutter speeds, 1/125th of a second or less, can capture objects in motion.
In addition to regulating the intensity of the light that reaches the film, the diaphragm aperture is also used to control the depth of field. Also called the zone of focus, depth of field refers to the area in which objects recorded in the picture will be sharply focused. Decreasing the size of the aperture increases the overall depth of field; widening the aperture decreases it. When great depth of field is desired—maximum sharpness of all points in the scene, foreground to background—a small aperture and slow shutter speed are used. Since the faster shutter speeds needed to capture motion require, in compensation, larger apertures, the depth of field is reduced. On many cameras, the lens ring contains a depth-of-field scale that shows the approximate sharp-focus zone for the different aperture settings.
B Camera Designs
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, twin-lens reflex (TLR), and rangefinder. Most SLR and rangefinder cameras use the 35-mm film format, while most TLR as well as some SLR and rangefinder cameras use medium-format film, that is, size 120 or 220.
B1 View Cameras
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 wooden base with a geared track on which two metal standards ride, one at the front and one at the 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 unparalleled control of perspective and focus.
B2 Rangefinder Cameras
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, they become more pronounced, making it difficult for the photographer to frame a scene or subject with certainty.
B3 Reflex Cameras
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° 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 on to 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. Minolta’s Maxxum 7000i has software “cards” which, when inserted in a slot on the side of the camera, expand the camera’s capabilities.
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 centre of the focusing screen.
B4 Design Comparisons
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 x 5 in 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 rugged 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 (the Apollo astronauts used Hasselblad TLRs on the Moon). 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, while still limiting photographer control.
C Camera Lenses
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 millimetres. Focal length is defined as the distance from the centre 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-mm photography, lenses with focal lengths from 20 to 35 mm 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 fisheye, lenses provide fields of view of 180° or more. A 6-mm fisheye lens made by Nikon has a 220° field of view that produces a circular image on film, rather than the normal rectangular or square image.
Lenses with focal lengths of 45 to 55 mm are referred to as normal lenses because they produce an image that approximates the perspective perceived by 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-mm camera, lenses with focal lengths of 85 mm 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.
D Artificial Light Sources
In the absence of adequate sunlight, photographers use artificial light to illuminate scenes, both indoors and outdoors. The most commonly used sources of artificial illumination are the electronic flash, or “strobe”; tungsten lamps called photofloods; and quartz lamps. Another once-popular light source, the flashbulb, a disposable bulb filled with oxygen and a mass of fine magnesium alloy wire that fired only once, is largely obsolete, having been replaced by inexpensive, economical electronic flash units.
The electronic flash (effectively a kind of stroboscope) consists of a glass quartz tube filled with an inert gas and a halogen gas at extremely low pressure. When high voltage is applied to the electrodes sealed at the ends of the tube, the gas ionizes and produces an intense burst of light of very short duration, a flash. Although large, special-purpose units can produce a flash of about 1/100,000 of a second, most produce flashes lasting from 1/5,000 to 1/1,000 of a second. Flash units must be synchronized with the shutter of the camera so that the burst of light covers the entire scene. Synchronization is achieved through an electrical connection between camera and flash unit, either a bracket mounted on top of the camera, called a “hot shoe”, or a cord called a “synch cord” that runs from the camera’s synchronization socket to the flash.
Automatic flash units are equipped with sensors, photocells that automatically adjust the duration of the flash for a particular scene. The sensor, which measures the intensity of the flash as it occurs, cuts off the light when adequate illumination is obtained. The dedicated flash, a newer type of automatic flash, is designed to function as a unit with a particular camera. The electronic circuitry of the flash and camera are integrated. The sensor is located inside the camera and gauges the amount of light at the film plane, allowing more accurate measurement of flash intensity.
Flash units vary in size from small camera-mounted units to large studio units. Generally speaking, the larger the unit, the greater the intensity of light produced. Camera-mounted flashes are adequate for illuminating small scenes, but to illuminate a large scene evenly, and with a single burst of light, a powerful studio unit is needed.
Photofloods, incandescent bulbs with filaments thinner than those used in ordinary light bulbs, provide continuous light. For normal colour rendition in colour photography, photofloods must be used with either tungsten-balanced film or a light-balancing filter. Quartz lighting, the standard of the television industry because of the great intensity of light produced and relative longevity of the bulbs when compared to tungsten sources, is also popular among still photographers.
E Exposure Meters
Serious photographers use exposure meters to measure the intensity of light in a given situation to determine the proper combination of shutter speed and diaphragm aperture. Four major types of meter are in use: incident light, reflected light, spot, and flash, although strictly speaking, spot meters are a type of reflected light meter, and flash meters can be of either incident or reflected type.
Incident light meters measure the intensity of light falling on a subject. To take an incident light reading, the meter is placed alongside the subject and pointed at the camera. Reflected light meters measure the intensity of light reflected by the subject. They are read with the meter at the camera, pointed towards the subject. Most incident light meters can also be modified for use as reflected light meters.
Spot meters measure reflected light in an area as little as 1°, whereas the types mentioned above cover a much broader angular range: from 30° to 50° for a reflected-light meter, to 180° for an incident light meter. Flash meters are designed to measure only the split-second bursts emitted by flash units. Combination meters are designed with incident-light, reflected-light, and flash-metering capability.
The simplest meters contain a photoelectric cell that generates a tiny electric current when exposed to light and moves a pointer across a scale. The meter is equipped with an adjustable dial indicating film speed. When the dial is aligned with the pointer, the meter shows the various combinations of shutter speeds and apertures that will produce equivalent exposures, and the camera can be set accordingly.
In some meters, a photoconductive cadmium sulphide cell serves as the light-sensitive element. Powered by a mercury battery, the cell is extremely sensitive, even under low-light conditions. A late-1980s innovation was the use of silicon diodes as the light receptor. Meters equipped with these have even greater sensitivity to light than cadmium sulphide cells.
For studio photography, a special meter that measures the colour temperature of light is often used. Different wavelengths of light correspond to particular temperatures, expressed in degrees Kelvin (K), and different kinds of lighting have their own specific colour temperatures. Colour-temperature meters allow precise measurement of the light emitted by various kinds of bulbs. This is essential for professional colour photography done indoors under artificial lighting because the colour temperature of fluorescent and incandescent bulbs varies from manufacturer to manufacturer, and the colour temperature of a bulb can also change with age.
Made of gelatin or glass, filters are used in front of a camera lens to alter the colour balance of light, to change contrast or brightness, to minimize haze, or to create special effects. In black-and-white photography, colour filters are used with panchromatic film to transmit light of the matching colour while blocking light of a contrasting colour. In a landscape photograph taken with a red filter, for example, some of the blue light of the sky is blocked, causing the sky to appear darker and thereby emphasizing clouds. Under a blue sky, a yellow filter produces a less extreme effect because more blue light is transmitted to the film. The No. 8 yellow filter is often used for outdoor black-and-white photography because it renders the tone of a blue sky in much the same way that the human eye perceives it.
Conversion filters, light-balancing filters, and colour-compensating filters are all widely used in colour photography. Conversion filters change the colour balance of light for a given film. Tungsten films, for example, are designed and balanced for the colour temperature of amber tungsten light. Exposed in daylight, they will produce pictures with a bluish cast. A series 85 conversion filter can correct this. Daylight film, on the other hand, balanced for sunlight at noon, which has a greater concentration of blue wavelengths than tungsten light, will have a yellow-amber cast when exposed under tungsten light. A series 80 conversion filter corrects this problem.
Light-balancing filters are generally used to make small adjustments in colour. These pale-toned filters eliminate undesirable colour casts or add a general warming hue. Colour-compensating (CC) magenta filters can balance greenish fluorescent light for daylight or tungsten film. Another type of filter, the polarizer, is used primarily to reduce reflection from the surface of shiny subjects. Polarizing filters are also used in colour photography to increase colour saturation.
IV 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; the image is called a print. Film is developed by treating it with a weak reducing alkaline chemical, 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 further reduce 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 that 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 thiosulphate, although potassium or ammonium thiosulphate 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 on to 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.
Colour prints from colour negatives are made either by the projection method or by contact printing. Prints from colour 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, colour 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 colour printing process, called dye-transfer, is considerably more complex and generally used only for professional work.
Positive colour transparencies and colour negatives are printed on papers with multilayer emulsions containing colour-forming agents. Examples of these are Fujichrome Type 34 process paper and Kodak Ektachrome, which are used for printing from colour transparencies; and Ektacolour, Fujicolour, and Agfacolour CN Type A, which are used for printing from negatives. These papers are developed in dye-forming solutions without reversal processing. When colour prints of this type are made, errors in exposure can be minimized by varying print exposure time. Colour 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 colour prints, a separate negative is prepared for each of three colours: red, green, and blue. These colour-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 colour 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 yellow dye, another in magenta, and the third in cyan. After immersion, each matrix is printed in turn on a special easel that ensures precise alignment, or registration, to form a full-colour image.
V RECENT TECHNOLOGICAL ADVANCES
A Advanced Photo System
In the early 1990s Kodak introduced a new line of cameras and film designed for amateur photographers. Called Advanced Photo System (APS), this technology challenges conventional 35-mm photography on several fronts. APS film is easier to load, since the APS film cartridge has no leader to thread into a take-up spool, and APS cameras are able to code magnetic information on to the exposed film that automated photo-finishing machines can read. According to Kodak, this results in a higher percentage of well-exposed prints than with standard 35-mm processing. Although APS film is a smaller format than 35-mm film, it is capable of results that nearly match the precision and sharpness of the older format. The system also allows the photographer to choose between three different print sizes—standard, wide-screen, and panoramic—at the time of taking the picture.
B Digital Photography
In the late 20th century, new technologies began to blur the lines between photography and other image-making systems. In some new forms of still photography, silver-halide emulsions were replaced by electronic methods of recording visual information. In 1981, the Sony Corporation introduced its still video camera the Mavica, based on an earlier industrial model, the ProMavica. Unlike the conventional video camera, which used magnetic tape, the Mavica recorded visual data—light reflected from objects in the scene photographed—on a floppy disk. The images were viewed on a monitor connected to the Mavica’s playback unit. Canon also entered the still-video-camera market. Its RC-470 camera required a still video player for viewing, but the Xap Shot, which recorded 50 still images, with 300 to 400 lines of resolution, on a 5-cm (2-in) floppy disk, did not require any special equipment. It could be connected directly to a television receiver. Paper prints of the recorded images could also be made, using a laser-driver computer printer.
Digitization of photographic images began 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) made it possible to manipulate the photographic image by means of specially developed computer programs. In the 1980s, the Scitex image-processing system enabled the user to move or erase elements in a photograph, change colours, fashion composite images from several photographs, and adjust contrast or sharpness. Adobe’s Photoshop application further refined these capabilities, becoming the standard image-editing tool of the print and Web design industries in the 1990s.
As the quality of digital imaging technology improves, digital photography has begun to replace conventional photographic technology, both professionally, in areas such as photojournalism, and among amateur enthusiasts. Digital cameras utilize a light-sensitive chip for image capture called a CCD (charge-coupled device), which is made up of sensors that gather colour and light information. This information is then converted into digital data—pixels. In a high-resolution, full-colour photograph taken with the digital equivalent of a professional 35-mm SLR camera, there will be as many as several million pixels. Some digital cameras are able to transfer their large picture files directly into a computer for storage. Others accept a disk or similar portable storage unit to achieve the same purpose. The original high-resolution image can later be reproduced in ink (in a magazine, for example) or as a conventional silver halide print.
Digital cameras aimed at the amateur photography market function much as point-and-shoot cameras do, with automatic focus, automatic exposure, and built-in electronic flash. Pictures from these cameras contain fewer pixels than those from a more expensive camera and are therefore not as sharp. After taking pictures, image files can be transferred to a home computer, stored on disk, or sent via e-mail.
VI 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.
A High-Speed Photography and Cinematography
Most modern cameras allow exposures with shutter speeds of up to 1/1,000 second. Shorter exposure times can be attained by illuminating the object with a short light flash. In 1931 the 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 shutter 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 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 the explosion of an atomic bomb.
Very 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 5,000 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.
B Aerial Photography
Special cameras are often equipped with several lenses and large film magazines and set in vibration-free mountings on aeroplanes. They are used in extensive land surveys for map-making, for studying the growth of cities for town planning, for detecting traces left by ancient civilizations, and for observing land use and the distribution of animal populations and vegetation. Cameras mounted in satellites are also used for such photography. A special application of aerial photography is military surveillance and reconnaissance; some reconnaissance satellites are equipped with cameras having objectives of long focal lengths that produce images, of very high resolution, on which cars, or even smaller objects, can be recognized. Advanced satellite photographic methods, which until recently were used almost exclusively by military, intelligence, and weather agencies, are increasingly being employed by geologists to uncover mineral resources and by news organizations to obtain instantaneous photographs of distant news events.
C Underwater Photography
Underwater cameras require a watertight housing with a glass or plastic window in front of the lens. Usually, during daytime, photographs can be taken at depths of up to 10 m (more than 30 ft). Greater depths require artificial light, such as an electronic flash or floodlight. The quality of the photographs depends on the clarity of the water; in water full of particles, the light reflected from the particles renders anything but close-ups impractical. Underwater photographers often use wide-angle lenses to compensate for the effect that anything under water appears 25 per cent closer than it is in reality, because the refractive index of water is greater than that of air. Recording the beauty of the underwater world with the camera is a popular activity of scuba-diving enthusiasts. Special underwater cameras in pressure-resistant housings are also used in deep-sea exploration.
D Scientific Photography
In scientific research, photographic plates and films are among the most important recording tools, not only because of their versatility but also because the photographic emulsion is sensitive to ultraviolet and infrared radiation, to X-rays and gamma rays, and to charged particles. Radioactivity, for example, was discovered because of the accidental blackening of photographic film. Many optical instruments, such as the microscope, the telescope, and the spectroscope, can be used to obtain photographs. Many other scientific instruments such as electron microscopes, oscilloscopes, and computer terminals are also equipped with devices to take photographs or with adapters that permit the use of a regular camera. In laboratory research, Polaroid cameras are often used to obtain quick results. An important research activity in particle physics is the study of thousands of photographs taken in the cloud or bubble chambers of particle detectors in order to find interactions between particles of interest. Tracks of charged particles can also be recorded directly on special films.
The photographic recording of X-ray pictures, called radiography, has become an important diagnostic tool in medicine. Radiography, using very energetic X-rays or gamma rays, is also employed to detect welding defects and structural defects in pressure vessels, pipes, and mechanical parts, especially those that are critical for safety reasons, as in nuclear power plants, aeroplanes, and submarines. In many cases the film, wrapped in a light-proof envelope, is simply applied against one side of the object, while the object is irradiated from the other side. The photographic recording of X-rays is also used in structural studies of crystalline materials. With the development of the laser, a technique called lensless photography, or holography, became available for producing three-dimensional images. Photography is also important in recording images of very small objects when allied to various types of microscope, such as the scanning electron microscope, the transmission electron microscope, and the atomic force microscope. These technologies have been very important in biological and medical research.
E Astronomical Photography
In no other field of science has photography played a more important role than in astronomy. By placing the photographic plate in the focal plane of a telescope, astronomers can obtain precise records of the locations and brightness of celestial bodies. By comparing photographs of the same region of the sky taken at different times, proper motions of certain objects such as comets can be detected. An important quality of the photographic plate for astronomy is its ability to record, by means of long time-exposures, astronomical objects too faint to be observed with the naked eye.
Recently, the sensitivity of photographic recording has been improved by image-enhancing techniques. In a process known as the photoelectric effect starlight liberates electrons on a photocathode that is placed in the focal plane of the telescope. The liberated electrons are directed to a photographic plate to form the image. Computer enhancement techniques create sharper, more detailed images from sometimes fuzzy and distant photographs from outer space. Computers digitize the photographic information and then reproduce it with greatly improved resolution. In a further refinement, charge-coupled devices (CCDs) dispense with a photographic plate altogether; individual photons are recorded electronically, and are distributed by a microprocessor along a series of picture elements (pixels) which when built up in rows, form an image which can be manipulated digitally by computer. The pixel in these pictures is thus analogous to the silver-halide grain in conventional photography in governing the resolution of the image. CCDs are the imaging technology used by the Hubble Space Telescope and space exploration probes such as Galileo, and are the prime system used by the world’s major terrestrial telescopes, such as the Keck telescopes. The reason for their dominance in this area is their superiority in registering very faint astronomical objects; their ability to record individual photons gives tham a light-collecting power far greater than even the most sensitive film.
Microfilming consists of photographically reducing images to a very small size. An early application was the photographing of bank cheques in the 1920s; now the technique is widely used to store information that would otherwise require too much space. For example, newspapers and magazines are photographed on roll film and can be displayed on desk-top projectors equipped with systems that permit the desired pages to be found quickly. Another application is the microfiche, a piece of 10 by 15 cm (4 by 6 in) film on which up to 70 frames, each corresponding to one page of text, can be stored. Each frame can be viewed individually on a desk-top projector. This system makes possible the storage of the entire catalogue of a library on a relatively small number of microfiches.
G Infrared Photography
With special dyes, photographic emulsions can be made sensitive to light in the invisible infrared portion of the spectrum. Infrared light cuts through atmospheric haze and enables clear photographs to be taken from long distances or high altitudes. Because any object radiates in infrared light, it can be photographed in complete darkness. Infrared photographic techniques are used wherever small differences in temperature, or in absorption or reflection capacities for infrared light, have to be detected. Some substances, particularly organic ones such as vegetation, reflect infrared light more strongly than other substances do; infrared films tend to reproduce as white the tones of green leaves and plants, especially if used in conjunction with a deep-red filter. Infrared film has many technical and military applications, including the detection of camouflage, which in the infrared photograph appears darker than the surrounding area. Infrared photography is also used in medical diagnosis, in the detection of forgeries in handwriting as well as in paintings, and for the study of deteriorated documents. It has been used, for example, in deciphering the Dead Sea Scrolls.
H Ultraviolet Photography
Normal film is sensitive to ultraviolet light. In one method of ultraviolet photography, an ultraviolet light source is used to illuminate the object, and the camera lens is provided with a filter that permits only the passage of ultraviolet light. The second method makes use of fluorescence caused by ultraviolet light; a filter used on the camera absorbs ultraviolet light and allows the passage of the fluorescent light. One important application of ultraviolet photography is the study of forged documents, because traces of erased writing become detectable in ultraviolet light.
In several processes used to produce photographic images in the ultraviolet range of the spectrum, plastics and other chemicals that react to ultraviolet light replace the silver-halide emulsion of conventional film. In one process, surface areas of a plastic substance exposed to ultraviolet rays harden in direct proportion to the amount of exposure, and removal of the unhardened areas leaves a raised photographic image. In other processes, a thin film of chemicals is suspended between plastic sheets. When exposed to ultraviolet rays, these chemicals emit gas bubbles, in amounts proportional to the exposure received in a given area. The bubbles expand and become visible on the application of heat to the sheets, creating a transparency in which the gas bubbles form the image. Another type of plastic, when heated, reacts chemically with the gas bubbles so that a stained positive image is obtained on the plastic sheets.
In the photochromic film developed by the National Cash Register Company in the United States, a dye sensitive to ultraviolet light is used. Because the dye has no grain structure, enormous enlargements can be made. For example, enlargements can be made from film on which a complete book is contained in an area the size of a postage stamp.
With an ease that is remote from the laborious difficulty of making the earliest photographs, domestic photography today presents an ever-growing variety of techniques for making images, spurred on by improvements in camera design and the application of electronic and digital technology. In parallel, scientific photography constantly enlarges and refines its scope, as new techniques improve image quality and utilize the whole range of the electromagnetic spectrum.