Photography
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.
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 (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
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
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
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.
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-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.
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 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.
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 Nicéphore Niépce.
About 1831 French painter Louis Jacques Mandé 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 Félix Abel Niépce 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 Lumière—after the
process developed by French inventors Auguste and Louis Lumière—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.