Single-lens reflex camera
From Free net encyclopedia
The single-lens reflex (SLR) is a type of camera that uses a movable 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 roof pentaprism or pentamirror to observe the image via an eyepiece, but there are also other finder arrangements, such as the waist-level finder or porro prisms.
Image:Slr-cross-section.pngThe shutter in almost all contemporary SLRs sits just in front of the focal plane. If it does not, some other mechanism is required to ensure that no light reaches the film between exposures. For example, the Hasselblad 500C camera uses an auxiliary shutter blind in addition to its in-lens leaf shutter.
This cross-section (side-view) of the optical components of an SLR shows how the light passes through the lens assembly (1), is reflected by the mirror (2) and is projected on the matte focusing screen (5). Via a condensing lens (6) and internal reflections in the roof pentaprism (7) the image appears in the eyepiece (8). When an image is taken, the mirror moves in the direction of the arrow, the focal plane shutter (3) opens, and the image is projected in the film (4) in exactly the same manner as on the focusing screen.
This feature separates SLRs from other types, as the user sees the image as it would be captured. This aids in accurately knowing the image before hand.
Since the technology became widespread in the 1970s, SLRs have become the main type of camera used by dedicated amateur photographers and professionals.
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History
Large format SLR cameras were first built in the early years of the 20th century but were not very popular. Although the Soviet GOMZ sport (1935) was the first 35 mm SLR, it was the Ihagee Kine-Exakta (1936) that was truly influential. Further Exakta models, all with waist-level finders, were produced up to and during World War II. Meanwhile, Zeiss developed the eye-level viewfinder and pentaprism in prototype form, but the war intervened; in 1949 it saw production as the Contax S.
It was the Japanese who developed the SLR further; Asahi was a pioneer in this, with 1952's Asahiflex. The Asahiflex IIB (1954) had the first auto-return mirror, while 1957's Asahi Pentax brought a fixed pentaprism, the right-hand thumb wind lever, and the overall control scheme of most manual-wind SLRs for the next 25 years or more. Other Japanese manufacturers - Nikon, Canon and Yashica introduced their first SLRs in 1959 (the F, Canonflex, and Pentamatic, respectively). The Nikon F was the most successful of these, and the camera that switched many professional photographers to the 35 mm SLR. It was highly modular, extremely well-built and versatile.
Through-the-lens (TTL) light metering came to the SLR in the early 1960s, with 1962's Topcon RE Super (spot metering) and 1964's Pentax Spotmatic (center-weighted average metering). Auto-exposure was next, introduced by Pentax in 1971's Electro Spotmatic and popularised with 1976's Canon AE-1, one of the best-selling cameras of all time. Full program auto-exposure soon followed. The 1970s and 1980s saw steadily increasing use of electronics, automation and minaturization, including integrated motor driven film advance with the Konica FS-1 in 1979, and motor rewind functions.
Autofocus was the next major SLR revolution. The first autofocus SLR was 1981's Pentax ME-F, but it was 1985's Minolta Maxxum 7000 that shifted the market, and most competitors followed. Minolta, Canon and Contax chose to bring out a new, incompatible camera and lens system for autofocus, while Nikon and Pentax chose a backward-compatible method that ensured owners of older lenses could continue to use them. In order to improve AF speed and reduce weight, LCD (liquid crystal display) panels, IC chips, microcomputers, and other electronic components with fixed lifetimes began to appear, offering greater convenience and capability at the cost of a fixed service life - once parts supplies were gone, the cameras could no longer be repaired, even by a skilled technician. Additionally, the rising costs of production, consumer demand for low prices, and first-generation electronics sometimes brought more frequent malfunctions.
Other manufacturers did not transition successfully to the autofocus era, although many camera makers continued to sell manual focus SLRs to photographers who did not need or want AF, notably Nikon, Pentax, Yashica, and Leica. Manual focus cameras, especially pro-level and mechanically operated models, also were more reliable in extreme conditions, and had a longer service life. Pentax, Leica and Cosina continue to produce manual-focus SLR cameras today.
The 1990s mostly saw further increases in automation and sophistication, though in consumer level cameras, this added complexity often meant more malfunctions and shorter life, particularly after the introduction of consumer-level digital cameras. Canon, Nikon, Pentax and Minolta have all introduced commercially successful digital SLR ranges compatible with their film SLR systems (though Konica-Minolta recently sold its SLR camera division to Sony who will continue manufacture), while Olympus has introduced a new digital-only SLR system, the Four Thirds system.
Format
SLR cameras have been produced for most film formats as well as digital formats. Most film SLRs use the 35 mm format, as this offers a good compromise between image quality, size, and cost. Medium format SLRs give a higher quality image when this is required. Digital SLRs (DSLRs) appeared on the market in the late 1990s and as of 2006 are used by many professional photographers as well as amateur enthusiasts. Early SLRs were built for large format photography, but this has largely died out. A small number of SLRs were built for the Advanced Photo System but this did not prove popular. SLRs were even built for film formats as small as 110, e.g. the Pentax Auto 110.
Common features
Other features found on many SLR cameras include through-the-lens (TTL) 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 enough 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.
Advantages
Many of the advantages of SLR cameras derive from viewing the scene through the taking lens. There is no parallax error, and exact focus can be confirmed by eye—otherwise hard for macro photography and when using telephoto lenses. The true depth of field may be seen by stopping down to the taking aperture, possible on all but the cheapest cameras. Because of the SLR's versatility, most manufacturers have a vast range of lenses and accessories available.
Compared to most fixed-lens compact cameras, the most commonly used and cheapest SLR lenses offer a wider aperture range and larger maximum aperture (typically f/1.4 to f/1.8 for a 50 mm lens). This allows photographs to be taken in lower light conditions without flash, and allows a narrower depth of field, which is useful for blurring the background behind the subject, making the subject more prominent. This is commonly used in portrait photography.
In the diagram the light path through the roof pentaprism is shown in simplified form. In fact an odd number of reflections are required to rectify the image on the ground glass screen. The first reflection from the moving mirror requires an additional three reflections inside the prism. Two only are shown.
Disadvantages
The most obvious disadvantage of the SLR is its inability to view the scene at the moment the shutter captures the image. (One camera, the Canon Pellix, solved this problem by using a stationary beam splitting mirror but was not a commercial success.) A second disadvantage is a normally greater weight and size than rangefinders of a similar technology level - the pentaprism and mirror box make the camera body larger. However, rangefinders have not advanced significantly since the 1970s, while modern SLRs use advanced automation, plastics, and electronics to be smaller - sometimes at the price of long-term durability and reliability. Plastic moving parts don't last as long as metal.
The SLR's space-consuming mirror movement makes for difficulty in constructing wide angle lenses; rear lens elements cannot be close to the film plane. Retrofocus designs are required for wide-angle lenses; these are complex, large, and comparatively poorer in image quality.
The reflex mirror must retract before the shutter can open, which introduces some delay. Autofocus systems on modern SLRs introduce further delay, especially in lower light. The mirror's movement also causes vibration and noise, a problem when using longer lenses and longer exposures. Technology has reduced but not eliminated this problem, which again is worse in larger formats. To combat this, higher-end cameras offer the ability to lock up the mirror before the shot is taken. This eliminates the vibration but blacks out the viewfinder.
The SLR user cannot see anything outside the taking frame through the viewfinder, while with most rangefinder systems, this can be done. This helps in certain kinds of photography. Only higher-end SLRs show the full frame; typical coverage is 90%. Print labs generally crop an equivalent area, so it is less of a problem than it might otherwise be.
In the digital arena, the DSLR cameras have two further drawbacks compared to Compact or Bridge digital camera: The back panel display is usually unable to function to assist with image composition. Movie modes are generally unavailable.
Notes
See also
- Category:SLR cameras
- Twin-lens reflex camera
- View camera
- Box camera
- Rangefinder camera
- Zeiss Ikoncs:Zrcadlovka
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