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The single-lens reflex camera, more commonly known by the abbreviation SLR, uses a mirror placed between the lens and the film to project the image seen through the lens to a matte focusing screen. Most SLRs use a pentaprism to observe the image via an eyepiece, but there are also other finder arrangements, such as the waist-level finder.
Since the technology became widespread in the 1970s, SLRs have become the main type of camera used by dedicated amateur photographers, and professionals.
The major advantage of single lens reflex cameras is the ability to determine what parts of the image are in focus by looking through the taking lens itself. Many modern SLRs offer additional focussing aids, including rangefinder devices and automatic focussing methods. To save weight, many modern SLRs use mirrors instead of a pentaprism. However, the resulting image is usually somewhat darker than the one from a pentaprism.
The SLR cameras also offer the ability for the photographer to quickly and simply change the lens, thus making them versatile. The same camera can be used for portrait, landscape, action, and close-up photography. Whilst some earlier non-SLR cameras offered this ability, the SLR immediately allows the photographer to see what the different lenses will capture.
SLR cameras also avoid the difficulty of parallax in close-up photography. Cameras with a separate optical viewfinder system show the scene from a different viewpoint from that of the camera lens. For photography at normal distances, this difference is not significant, but where the gap between the viewfinder and lens is a significant fraction of the distance to the objects in the scene to be photographed, this difference in viewpoint may mean that the image recorded is quite different from that seen through the viewfinder.
The majority of SLR cameras use 35mm film, as this format offers a good compromise between image quality, size, and cost for most amateur and some professional applications. Professional photographers often use medium format SLRs for work where high image quality is important. Digital SLRs have also appeared on the market and are now the camera of choice for most newspaper photographers, and are starting to be widely adopted by amateur enthusiasts. Single-lens reflex cameras using the APS film format were not popular.
Other features found on many SLR cameras include through-the-lens metering and sophisticated flash control. Many models on the market today actually measure the light that bounces off the film, and close the shutter when the picture has had enought exposure. Likewise, they can send out several short bursts of flash, determine the amount that comes back from the scene, then send out just the right amount of energy for a perfectly exposed photograph. Sophisticated cameras can even make it easy for the photographer to balance flash and available light for the desired look. While these capabilities are hardly unique to the SLR, manufacturers included them early on in the top models, whereas the best rangefinder cameras adopted such features later.
The pros of SLR camera are:
Absence of parallax
Exact focusing, esp. important for macro and telephoto photography
Ability to check the depth of field (a feature often omitted on low-end SLRs)
Vast range of interchangeable lenses (this feature is generally available also on high end rangefinder cameras, but the SLR concept is perfect for it)
The cons of SLR camera are:
Due to mechanical complexity and the bulky finder, the SLR camera is larger than a rangefinder camera for a given film size.
Latency; the mirror needs time to move before the film is exposed. On modern SLRs, latency is generally increased by autofocus, and can be a real inconvenience.
Noise and vibrations, mainly due to the mirror movement.
Dark viewfinder when using low aperture lenses.
History
Graflex produced SLR cameras as early as 1909. The modern 35mm SLR concept was introduced by the Exakta in 1936. Significant SLRs that set a standard were the Hasselblad, the Asahi Pentax, the Minolta SRT 101, the Nikon F and the Minolta Maxxum 7000.
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