Except for pinhole cameras, which focus the image on film or viewing surface 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 sensor, determines the angle of view and the size of objects as they appear on the imaging surface. The image is focused on that surface (the focal plane) by adjusting the distance between the lens and the surface. In most 35-mm cameras (among the most widely used of modern optical, or film, cameras) and digital cameras this is done by rotating the lens to move it closer to or farther from the focal plane. 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 focal plane.
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 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 or imaging sensor, 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 or imaging sensor 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 or imaging device. Digital cameras typically have a liquid crystal display (LCD) screen and may have an electronic or optical viewfinder as well; some digital cameras have a viewfinder only. The LCD screen allows the photographer to see the image's content before the picture is taken and after, facilitating the deletion of unwanted pictures. An electronic viewfinder effectively reproduces in miniature the image displayed on the LCD screen; the image can be augmented by the camera software in various ways to aid the photographer.
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 mechanical 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 or imaging sensor and has one or two cloth or metal curtains that travel vertically or horizontally across the film frame. Digital cameras may use a mechanical shutter (normally a focal-plane shutter) or an electronic shutter, or a combination of the two; the digital camera in a smartphone uses an electronic shutter. There are two types of electronic shutters. One, known as a global shutter, transfers the data being recorded by the imaging sensor all at once; the other, a rolling shutter, transfers the data line by line. 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 or imaging sensor being used) for a good exposure can be obtained.
Most modern 35 mm film cameras, both reflex and rangefinder 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 can be used to produce special affects and compensate for difficult lighting conditions (more important in film photography in some instances); flash mechanisms for supplying light; and monopods and tripods, for steady support. Digitial versions of reflex and rangefinder models (the former of which is much more common) also have similar features.
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. Formerly very popular were compact 35-mm rangefinder cameras; 126 cartridge cameras; and the subminiature cameras, including the 110
The smaller, pocket-sized, automatic cameras of the Advanced Photo System (APS), introduced in 1996 and now obsolete, were unique in that they were part of an integrated system. Using magnetic strips on the film to communicate with the photofinishing equipment, the camera could report shutter speed, aperture setting, and lighting conditions for each frame to the computerized photofinishing equipment, which could then compensate to avoid over- or underexposed photographic prints. Basic features of the APS cameras were 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,
Digital cameras are similar in many respects to film cameras but capture the image electronically by means of a semiconductor-based imaging device rather than via a photographic emulsion. The imaging device uses either a complementary metal-oxide semiconductor (CMOS) image sensor or a charge-coupled device (CCD). Generally CMOS image sensors are cheaper and faster than those based on CCDs, but CCDs typically produce higher quality images. Because good quality digital cameras are now incorporated in cellphones, especially smartphones, the camera phone has superseded previously popular consumer cameras such as the
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 24 megapixels. With more megapixels, more picture detail may be captured, resulting in sharper, larger images, but picture quality also benefits from the use of a larger imaging sensor with larger pixels, which will have more light-gathering capacity, so a digital camera with somewhat fewer but larger pixels may produce better images than one with more but smaller pixels. 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 solid-state flash memory devices. The images can be transferred to a computer for viewing and editing or may be viewed on the camera's LCD screen. Digital cameras typically also have the ability to record video, but have less storage capacity and fewer video features than camcorders.
Camera phones have become the ubiquitous progeny of the marriage of microelectronics and digital technology, common in relatively simple cellular telephones and a defining and essential feature of smartphones, which really are small computers masquerading as cellphones. Like digital cameras, they can record both still images and video, and though they are limited by fixed apertures, relatively small image sensors, digital zoom, and reliance on an LCD screen as a viewfinder, CMOS-based smartphone cameras produce good quality images. The phone can be used to view the image or immediately send the picture to another camera phone or computer via the Internet, the telephone network, and the like, 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 omnipresent camera phone is available for impromptu or unanticipated photographic opportunities, including the always essential
See also photography, still.
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