Camera

This article explains how a camera takes pictures and discusses various types of cameras. For a discussion of the history and parts of a camera and how to take pictures, see the article on Photography.

In addition, most cameras have a viewfinder, a sighting instrument that the photographer looks through to frame the subject. Most cameras also have a film advance. With this device, the photographer winds the film through the camera to put unexposed film in position to take a picture. After each picture has been taken, the film advance moves the exposed film out of the way. The film advances automatically in many cameras.

Controlling the light. To take good pictures, a camera must let the proper amount of light reach the film. Two devices control the light: (1) the shutter, a movable set of blades or curtains that open to let light into the camera, and (2) the diaphragm, an adjustable ring that expands or contracts the aperture, the opening through which light enters. The speed of the shutter's opening and closing determines how long the film is exposed to light. The size of the aperture determines how much light reaches the film during that time.

Many cameras have adjustable shutter speeds, ranging from 30 seconds to 1/12,000 of a second. Many cameras also have an adjustable aperture. The various sizes of an aperture are often called f-stops.

The settings for shutter speed and aperture are interrelated. Suppose you wanted to "freeze" a fast-moving object. You would need a fast shutter speed--in other words, a short exposure time. To let in enough light during this brief time, you would need a large aperture.

Now, suppose you wanted to photograph a sweeping landscape. To bring into sharp focus all parts of the landscape that were at various distances from the camera, you would need a small aperture. To let in enough light through this narrow opening for proper exposure, you would need a slow shutter speed.

Providing additional light. Many cameras have a built-in or attachable electronic flash unit that provides any additional light needed to take a picture. The flash unit provides a short burst of light synchronized with the opening of the shutter.

Automatic controls. Many cameras also have built-in devices that perform various parts of the picture-taking process automatically. A tiny built-in computer called a central processing unit (CPU) or microprocessor determines the correct settings for picture taking. Sensors inside the camera tell the CPU what speed of film is in the camera and how bright the light is. The CPU processes this information and either adjusts the camera settings itself or displays the information so that the photographer can set the camera by hand.

The two chief automatic features are (1) automatic exposure control and (2) automatic focusing.

Automatic exposure control, also called autoexposure, helps ensure proper exposure of the film. A feature called aperture-priority automation enables the photographer to set the f-stop, while the camera chooses the shutter speed. Shutter-priority automation does the opposite. It lets the photographer set the shutter speed while the camera selects the aperture for proper exposure. Many cameras offer programmed exposure, in which the camera chooses both the shutter speed and the f-stop.

All automatic exposure systems rely on readings from a built-in light meter. The camera's CPU interprets these readings according to a metering pattern, a set of instructions that tells the CPU how to react to different amounts of brightness in specific parts of the scene. Some cameras offer a choice of metering patterns. Using the readings and a metering pattern, exposure systems can compensate for much unevenness in lighting, such as a scene with a bright background behind a dark subject.

Automatic focusing, also called autofocus, adjusts the lens focus automatically to provide sharp photographs. There are two chief types of automatic focusing systems: (1) active and (2) passive.

An active system bounces an infrared (heat) ray or an ultrasound wave off the subject--that is, whatever object is within brackets or a box in the viewfinder. Sensors measure the angle at which the infrared beam returns to the camera or how long it takes the ultrasound wave to return. The system uses the measurement to determine how far away the subject is and where the lens should focus.

A passive autofocus system analyzes the sharpness of the image entering the lens. In most systems, sensors behind the lens measure the contrast of lines or edges in the subject. This contrast is highest when the camera is sharply focused on the subject. The system adjusts the focus to achieve the maximum contrast.

Some autofocus systems can sense if a subject is moving. These systems make adjustments so that the subject is in focus during the instant the shutter is open.

In general, a fixed-focus camera can take satisfactory photographs in ordinary daylight but not in dim light, because its aperture does not admit much light. The camera may produce a blurred picture if the subject is moving or is less than 6 feet (1.8 meters) away. Many fixed-focus cameras can take flash pictures.

Disposable or recyclable cameras are a kind of fixed-focus camera that combines a plastic lens, a shutter, and film in one small box. The entire camera is brought to the photo lab when the roll of film has been exposed.

Single-lens reflex cameras appeal to skilled amateur photographers and to professional photographers. The camera's name refers to its viewing system. The photographer views the subject through the camera lens rather than through a separate viewing lens. A mirror between the lens and the film reflects the image to the viewfinder. When the shutter release button is pressed to take a picture, the mirror lifts out of the way to allow the light to expose the film. Thus, the photographer sees almost the exact image that is recorded on the film. Most single-lens reflex cameras use 35-millimeter film. The photographer can adjust the focus, select the shutter speed, and control the opening of the diaphragm. Many new models can also adjust the focus and control the light exposure automatically.

The standard lens of the single-lens reflex camera can be replaced by special-purpose lenses that change the size and depth relationship of objects in a scene. These lenses include wide-angle lenses, telephoto lenses, and zoom lenses. A wide-angle lens provides a wider view of a scene than a standard lens does. A telephoto lens makes objects appear closer. A zoom lens combines features of standard, wide-angle, and telephoto lenses.

Twin-lens reflex cameras have a viewing lens directly above the picture-taking lens. The image in the viewfinder appears on a flat screen built into the top of the camera. Photographers find such a screen helpful in composing a picture. Photographers do not hold the viewfinder to the eye, as they do with a fixed-focus, point-and-shoot, or single-lens reflex camera. They usually hold it at the chest or waist and look down into the viewfinder. The image appears reversed from left to right. Most such cameras use film that produces negatives measuring 21/4 by 21/4 inches (5.7 by 5.7 centimeters).

Point-and-shoot cameras have many automatic features that make them easy to use. Electronic devices inside the cameras automatically adjust the focus, set the light exposure and the shutter speed, and advance and rewind the film. A built-in electronic flash automatically supplies light when too little light reflects off the subject. Point-and-shoot cameras use 35-millimeter film. These cameras are popular among amateur photographers. Some of them have a zoom lens.

View cameras are the largest and most adjustable type of camera. Most have an accordionlike body, with a replaceable lens in front. They have a large viewing screen instead of a viewfinder. Most models have an adjustable diaphragm and shutter speed. View cameras must be mounted on a stand for efficient operation.

A photographer focuses a view camera by moving the lens end or the back end of the camera forward or backward to produce a sharp image on the viewing screen. Adjustments in the tilt of the camera give the photographer great control over the image. A view camera can provide artistic distortions of a subject more effectively than any other kind of camera.

Many professional photographers use a view camera for portraits and other subjects. A view camera uses sheets of film that range from 21/4 by 31/4 inches (57 by 83 millimeters) to 11 by 14 inches (279 by 356 millimeters).

Instant cameras use film that provides a print without first being developed into a negative. The cameras produce a print 15 seconds to 2 minutes after the photographer takes a picture. The time varies according to the camera and the type of film. Instant cameras use film that provides pictures ranging in size from 27/8 by 35/8 inches (73 by 92 millimeters) to 20 by 24 inches (508 by 610 millimeters). Special types of film for instant cameras also provide negatives. Some instant cameras can take flash pictures and focus automatically.

Electronic cameras, including digital cameras, create pictures that can be viewed on a television screen or transferred to a computer. The lens in most electronic cameras focuses light on a light-sensitive mechanism called a charge-coupled device, or CCD. The CCD changes the light into electronic signals. The electronic pictures can then be stored on floppy disks used in computers, on electronic storage devices called memory cards, or on a hard disk. With additional equipment, electronic images can also be sent over telephone lines or printed on paper.

Motion-picture cameras take pictures that re-create the motion of a subject when they are viewed. Professional moviemakers generally use large cameras that take 70-, 35-, or 16-millimeter film. Most amateurs record on 8-millimeter film that is called super-8. Today, many amateur moviemakers use portable video cameras called camcorders. These cameras convert light reflected by the subject into electronic signals that are recorded on magnetic tape. Most movie cameras and camcorders can record sound at the same time as they record images. Most of them also have a zoom lens.

Stereo cameras produce images that seem to have depth. One kind of stereo camera has two identical picture-taking lens systems with matched shutters. This camera takes two pictures of the same subject at the same time--one picture through each lens system--but from slightly different angles. When viewed through special glasses or a device called a stereoscope the two pictures blend into one image that seems to have depth.

Contributor: Lawrence R. White, B.S., Technical Director, Popular Photography magazine.

Related Articles in include:

Camera lucida; Camera obscura; Lens; Light meter; Photography; Stereoscope.

Jervis, Alastair. Camera Technology. Bookwright, 1991. For younger readers.

Shipman, Carl. How to Select and Use ... HP Bks., 1985-. Each title covers a certain camera brand, such as How to Select and Use Canon SLR Cameras. Rev. ed. (1987).

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