Types of Cameras
A camera is a device that directs an image focused by a lens or other optical system onto a photosensitive surface housed in a light-tight enclosure. In this very basic sense, these components perform the same function today that they did when photography was invented nearly 150 years ago. In simple cameras the lens is generally of the fixed-focus variety: no provision is made to focus on objects at varying distances from the camera. More complicated cameras have a system to achieve good focus that is manually or automatically actuated, in order to vary the lens-to-focal-plane distance. (The focal plane is the point behind the lens where the image comes into focus.)
The photographic surface used in modern cameras is almost exclusively light-sensitive film. Flexible roll film may be housed in a cassette or on a paper-backed spool. A gear mechanism built into the camera advances the film between exposures. On professional, large-format cameras the film is a fairly stiff sheet that is carried in a holder to be inserted into the focal-plane area after the image has been focused.
Cameras are manufactured in a variety of types and sizes. Miniature instruments producing incredibly small images are used in medical research. Commercial portrait studios may use large-format view cameras that produce a film image as large as 11 x 14 inches.
The electronics revolution has had an immense impact on camera design, making possible instruments of remarkable sophistication in almost every price range.
The Box Camera
For more than seven decades the box camera was the instrument of choice for the casual amateur photographer. Inexpensive and simple, it was, nevertheless, capable of excellent results under many conditions. Box cameras were normally fitted with a single-element lens, a limited range of aperture control, and a single-speed leaf shutter.
The Folding-Roll Film Camera
Second in popularity only to the box camera, the folding camera was manufactured in a variety of formats. Basically, though, it was a box camera whose lens was incorporated into a movable bellows that could slide back and forth on a rail, allowing the lens to change focus. Lenses and shutters were often one-piece units. More-elaborate models were first-rate instruments with high-quality optical systems and precision shutters. Many were fitted with coupled rangefinders. The most significant advantage they had over the box camera, however, was their compact design when folded, which made them easier to pack and transport.
There has been something of a minor renaissance in folding-roll film cameras in recent years, with the appearance of several new professional instruments. They are appreciated for their large negative size and compact design.
Single-Lens Reflex Cameras
One of the most popular designs available today, the single-lens reflex (SLR) both views and photographs through one lens. Light passing through the lens is reflected by a mirror and brought to focus on a ground glass. The mirror causes a reversal of the image seen on the ground glass, but the addition of a pentaprism mounted over the ground glass allows the camera to be used at eye level, with the image seen upright and in proper left/right orientation. An instant before the exposure is made, the mirror swings upward, and the shutter is activated. A single control cocks the shutter for the next exposure, advances the film, and returns the mirror to focusing position.
Instant Cameras
An instant camera will produce a finished print in from 20 seconds to about 4 minutes. The film, after exposure, is passed between two stainless steel rollers inside the camera. These rupture a chemical pod on the film and spread developing agent evenly over the film's surface. In the original Polaroid system it was necessary for the user to peel the finished print from the base material. Professional Polaroid films, both color and black and white, are still developed in this manner. With the Polaroid SX-70 (1972), however, the developing picture is ejected from the camera, and the film reaches its final development in full daylight. The process is completed in about 4 minutes. Later instant cameras use a new focusing system employing an ultra-high-frequency sound emitter. An electronic circuit in the camera measures the time required for the sound to be reflected back from the object being photographed. This time measurement is converted into a measurement of distance, and an electrical mechanism coupled to the focusing circuit sets the lens for the proper exposure.
In 1993, Polaroid introduced a single-lens reflex model that does not eject shots like other instant cameras but instead transports the developed picture to a viewing area and container on the camera back.
Disposable Cameras
A disposable camera is a cardboard box containing a roll of film and a lens along with a simple viewfinder, a button for shooting, and a film advance mechanism. Like the earliest box cameras, the lens is fixed in focus--that is, its aperture is small enough so that anything more than about four feet away will photograph in focus. After the user has shot the full roll of film, the entire camera is returned to a processing lab, the film is developed and printed, and the box and its fittings returned to the camera maker who will use it to make new cameras.
Electronic Imaging
In an electronic still camera a cluster of light-sensitive CHARGE-COUPLED DEVICES (CCDs) is placed at the focal plane, where the film in a conventional camera would be positioned. Each light sensor on the CCD is called a pixel. In early electronic cameras, the pixels converted light into an analog, electrical signal. More advanced electronic cameras convert the light into digital code. Signals or code are recorded on a magnetic disc inside the camera. Images can be previewed immediately on a television or computer screen and adjustments made in the focus and exposure. Full-color prints, made on sophisticated laser and thermal printers, are often indistinguishable from the prints produced in a film-developing lab. All digitized images are manipulable using IMAGE PROCESSING techniques, and the digital data that comprises an electronic photo is permanently storable on computers and photo CDs.
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