Except for pinhole cameras, which focus the image on the film through a tiny hole, all other cameras use a lens for focusing. The focal length of a lens, i.e., the distance between the rear of the lens (when focused on infinity) and the film (or imaging device), determines the angle of view and the size of objects as they appear on the imaging surface. The image is focused on that surface by adjusting the distance between the lens and the surface. In most 35-mm cameras (among the most widely used of modern optical cameras) and digital cameras this is done by rotating the lens, thus moving it closer to or farther from the film or imaging device. With twin-lens reflex and larger view cameras, the whole lens and the panel to which it is attached are moved toward or away from the film.
To view the subject for composing (and, usually, to help bring it into focus) nearly every camera has some kind of viewfinder. One of the simplest types, employed in most view cameras, is a screen that is placed on the back of the camera and replaced by the film in making the exposure. This time-consuming procedure is avoided in the modern 35-mm single-lens (and other) reflex cameras by placing the screen in a special housing on top of the camera. Inside the camera, in front of the film plane, there is a movable mirror that bounces the image from the lens to the screen for viewing and focusing, and then flips out of the way when the shutter is tripped, so that the image hits the film instead of the mirror. The mirror returns automatically to place after the exposure has been made. In rangefinder cameras the subject is generally viewed by means of two separate windows, one of which views the scene directly and the other of which contains an adjustable optical mirror device. When this device is adjusted by rotating the lens, the image entering through the lens can be brought into register, at the eyepiece, with the image from the direct view, thereby focusing the subject on the film. Digital cameras have an optical viewfinder, a liquid crystal display (LCD) screen, or both. Optical viewfinders are common in point-and-shoot cameras. An LCD screen allows the user see the photograph's content before the picture is taken and after, allowing the deletion of unwanted pictures.
The speed of a lens is indicated by reference to its maximum opening, or aperture, through which light enters the camera. This aperture, or f -stop, is controlled by an iris diaphragm (a series of overlapping metal blades that form a circle with a hole in the center whose diameter can be increased or decreased as desired) inside the lens. The higher the f -stop number, the smaller the aperture, and vice versa.
A shutter controls the time during which light is permitted to enter the camera. There are two basic types of shutter, leaf-type and focal-plane. The leaf-type shutter employs a ring of overlapping metal blades similar to those of the iris diaphragm, which may be closed or opened to the desired degree. It is normally located between the lens elements but occasionally is placed behind or in front of the lens. The focal-plane shutter is located just in front of the film plane and has one or two cloth or metal curtains that travel vertically or horizontally across the film frame. By adjusting the shutter speed in conjunction with the width of aperture, the proper amount of light (determined by using a light meter and influenced by the relative sensitivity of the film being used) for a good exposure can be obtained.
Most of today's 35-mm cameras, both rangefinder and reflex models, incorporate a rapid film-transport mechanism, lens interchangeability (whereby lenses of many focal lengths, such as wide-angle and telephoto, may be used with the same camera body), and a built-in light meter. Many also have an automatic exposure device whereby either the shutter speed or the aperture is regulated automatically (by means of a very sophisticated solid-state electronics system) to produce the "correct" exposure. Accessories include filters, which correct for deficiencies in film sensitivity; flash bulbs and flash mechanisms for supplying light; and monopods and tripods, for steady support.
Simple box cameras, including cameras of the Eastman Kodak Instamatic type, are fixed-focus cameras with limited or no control over exposure. Twin-lens reflex cameras use one lens solely for viewing, while the other focuses the image on the film. Also very popular are compact 35-mm rangefinder cameras; 126 cartridge cameras; and the subminiature cameras, including the 110 "pocket" variation of the Instamatic type and the Minox, which uses 9.5-mm film. Other categories in use include roll- and sheet-film single-lens reflex (SLR) cameras that use 120 and larger size films; self-processing Polaroid cameras (see Land, Edwin H.); press cameras and view cameras that use 21/4 × 31/4 in., 4 × 5 in., 5 × 7 in., 8 × 10 in., and 11 × 14 in. film sizes; stereo cameras, the double slides from which require a special viewer; and various special types such as the super wide-angle and the panoramic cameras. (The numbers 110, 120, and 126 are film-size designations from the manufacturer and do not refer to actual measurements.) Digital cameras are essentially no different in operation but capture the image electronically rather than via a photographic emulsion.
The smaller, pocket-sized, automatic cameras of the Advanced Photo System (APS), introduced in 1996, are unique in that they are part of an integrated system. Using magnetic strips on the film to communicate with the photofinishing equipment, the camera can communicate shutter speed, aperture setting, and lighting conditions for each frame to the computerized photofinishing equipment, which can then compensate to avoid over- or underexposed photographic prints. Basic features of the APS cameras are drop-in loading, three print formats (classic, or 4 by 6 in.; hi vision, or 4 by 7 in.; and panoramic, or 4 by 11.5 in.) at the flick of a switch, and auto-focus, auto-exposure, "point-and-shoot" operation.
Digital cameras have several unique features. Resolution is made up of building blocks called pixels, one million of which are called a megapixel. Digital cameras have resolutions ranging from less than one megapixel to greater than seven megapixels. With more megapixels, more picture detail is captured, resulting in sharper, larger prints. Focus is a function of "zoom." Most digital cameras have an optical zoom, a digital zoom, or both. An optical zoom lens actually moves outward toward the subject to take sharp close-up photographs; this is the same kind of zoom lens found in traditional cameras. Digital zoom is a function of software inside the camera that crops the edges from a photograph and electronically enlarges the center portion of the image to fill the frame, resulting in a photograph with less detail. Some models also have a macro lens for close-ups of small, nearby objects. Storage of digital photographs may be in the camera's internal memory or in removable magnetic cards, sticks, or disks. These images can be transferred to a computer for viewing and editing or may be viewed on the camera's liquid crystal display. Digital cameras typically also have the ability to record video, but have less storage capacity and fewer video features than camcorders.
The marriage of microelectronics and digital technology led to the development of the camera phone, a cellular telephone that also has picture- (and video-) taking capability; smartphones, which integrate a range of applications into a cellphone, also typically include a camera. Some such phones can immediately send the picture to another camera phone or computer via the Internet or through the telephone network, offering the opportunity to take and share pictures in real time. Unlike the traditional camera, and to some extent the equivalent digital camera, which are used primarily for scheduled events or special occasions, the camera phone is available for impromptu or unanticipated photographic opportunities.
See also photography, still.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.