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