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The 'single-lens reflex' ('SLR') is a type of camera that uses an automatic mirror system placed between the lens and the film plane to focus and project the image seen through the lens, through a matte focusing screen which then diffuses the light, allowing it entrance into the roof pentaprism where the light is reflected and then directed to the photographer's eye.

Contents

Optical Components of an SLR Camera

Shutter Mechanisms

Focal Plane Shutter

Leaf Shutter - Two Types

Rotary Shutter

Further Developments

History

Through The Lens Light Metering (also known as "behind-the-lens metering"

Semi-Automatic and Fully-Automatic Exposure Capabilities

Autofocus

Digital SLRs

Format

Common features

Advantages

Disadvantages

See also

References

External links and sources

Optical Components of an SLR Camera

The cross-section (side-view) of the optical components of a typical SLR camera 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 upwards from its critical 45 degree angle in the direction of the arrow, the focal plane shutter '(3)' opens, and the image is projected onto the film or sensor '(4)' in exactly the same manner as on the focusing screen.
This feature distinguishes SLRs from other cameras, as the photographer sees the image composed exactly as it will be captured on the film or sensor (see Advantages below).
===Pentaprisms and Penta-Mirrors===
Most SLRs use a roof pentaprism or penta-mirror to direct the light to the eyepiece, but there are other finder viewing capabilities, such as the waist-level finder, the interchangeable sports finders used on the Canon F1, F1n and new F1; the Nikon F, F2, F3, F4 and F5; the Pentax LX; and the porro prism system used in the Olympus Pen F, the Pen FT, the Pen FV half-frame 35mm SLR cameras, and the digital E-300 and E-330 cameras. A photographer can also purchase a right-angle finder which slips onto the eyepiece of most SLR's and D-SLR's and allows viewing sans a waist-level finder.

Shutter Mechanisms

Focal Plane Shutter

Almost all contemporary SLRs use a focal plane shutter located in front of the film plane, which prevents light from reaching the film, even if the lens is removed, except when the shutter is actually triggered during the exposure. There are various designs for focal plane shutters. Typical focal plane shutters designed in the fifties and continuing through part of the 1990's consisted of cloth material and the shutter moved horizontally. Other focal plane shutters such as the Copal, travelled vertically, were made of titanium foil and were metal-bladed, resulting in faster flash synchronization. Certain horizontally travelling focal plane shutters were also constructed of titanium foil as is the case with the Nikon F, F2, F3, F4, F5 and F6 35mm SLR cameras, and the Canon F-1 camera series. A unique focal plane shutter design was the rotary shutter used in the Olympus Pen half-frame 35mm camera system, and this shutter allowed flash synchronization up to its top shutter speed of 1/500 of a second.

Leaf Shutter - Two Types

Another shutter system is the leaf shutter, whereby the shutter is constructed of diaphragm-like blades and can be situated either between the lens or behind the lens. If the shutter is part of a lens assembly some other mechanism is required to ensure that no light reaches the film between exposures. The leaf shutter can either be situated between the lens or behind the lens assembly.
An example of a behind-the-lens leaf shutter is found in the 35mm SLR's produced by Kodak, with their Retina Reflex camera line; Topcon, with their Auto 100; and Kowa with their SE-R and SET-R reflexes.
An example of between-the-lens leaf shutters, this time discussing medium format SLR's would be the manufacturer, Hasselblad with their 500C, 500CM, 500 EL-M (a motorized Hasselblad) and other models (producing a 2 1/4" square negative {or 6cm x 6cm metric}) using an auxiliary shutter blind situated behind the lens mount.
The now discontinued Zenza-Bronica camera systems such as the ETRs, the ETRs'i (both producing a 6 cm. x 4.5 cm. image), the SQ and the SQ-AI (producing a 2 1/4" or 6 x 6 cm. image like the Hasselblad), and the Zenza-Bronica G system (2 1/4" x 2 3/4" inch or 6 cm. x 7 cm.) also used between-the-lens leaf shutters in all of their lenses.
Thus, anytime a photographer purchased a lens for the Hasselblad, with the exception of the older Hasselblad 1000f and other focal-plane shutter Hasselblads; or the Zenza-Bronica ETR, ETR-S, ETR-Si, the SQ, the SQ-Ai, the SQ-B, and the G series (with the exception of older Bronica cameras such as the Bronica Deluxe, the Bronica S, the Bronica S2), or the bigger system Mamiya RB-67, RB-67 Pro, RB-67 Pro S (mechanical) and RZ (electronically controlled shutter) cameras, that lens included a leaf shutter in its lens mount.
Because leaf shutters synchronized electronic flash at all shutter speeds especially at fast shutter speeds of 1/500 of a second or faster, cameras utilizing leaf shutters were more desired by studio photographers who used sophisticated electronic flash systems.
Some manufacturers of medium-format 2 1/4" SLR cameras also made leaf-shutter lenses for their focal plane shutter models. Rollei made at least two such lenses for their Rolleiflex SL-66 medium format, 2 1/4" camera, which was a focal-plane shutter SLR. Rollei later switched to a camera system of leaf-shutter design, (i.e., the 6006 and 6008 reflexes to name a few) and their current medium-format SLR's are now all of the between-the-lens shutter design.

Rotary Shutter

One unusual design, the Olympus Pen half-frame 35mm SLR system, manufactured by Olympus, used a rotary focal plane shutter mechanism which was extremely simple and elegant in design, and enabled the photographer to synchronize electronic flash at all shutter speeds, including the shutter's limit to 1/500 of a second. The camera started out meterless with the introduction of the Olympus Pen F; this camera required a two-stroke advance of the advance lever. The later models, the Olympus Pen FT and the Olympus Pen FV only required a single stroke of the film advance lever to position the film to the next frame. The Olympus Pen FT has a behind-the-lens metering system and was composed of a number of impressive optics from a 20mm lens up to an 800mm catadioptric (mirror) telephoto lens. The system included 'fast' lenses such as the 38mm f/1.8 Zuiko, the 40mm f/1.4 Zuiko and the 42mm f/1.2 Zuiko, all considered 'normal' lenses for this format. Olympus also made a 38mm f/3.5 macro lens, and a bellows extension attachment. There were also various lens adapters and other accessories for this camera.
As a further, minor note on rotary shutters, only one other 35mm camera system used a rotary shutter, and this camera system was the Robot Royal cameras, all of which were rangefinder 35mm cameras. Some of these cameras were full-frame; some were half-frame, and at least one Robot camera produced an unusual square-sized image on the 35mm frame.

Further Developments

Since the technology became widespread in the 1970s, SLRs have become the main type of camera used by dedicated amateur photographers and professionals, though some photographers of static subjects (landscapes, commercial subjects, architecture) prefer view cameras[1] because of the capability to control perspective. With a triple-extension bellows 4" x 5" camera such as the Linhof SuperTechnika V, the photographer can correct certain distortions such as 'keystoning', where the image 'lines' converge (i.e., photographing a building by pointing a typical camera upward to include the top of the building). Perspective correction lenses are available in the 35mm and medium formats to correct this distortion with film cameras, and it can also be corrected after the fact with photo software when using digital cameras. The photographer can also extend the bellows to its full length, tilt the front standard and perform photomacrography (commonly known as 'macro photography'), producing a sharp image with depth-of-field without stopping down the lens diaphgram.

History

Large format SLR cameras were probably first sold in 1884.^[1] The Ihagee Kine-Exakta was the first 35 mm SLR and it was truly influential. Further Exakta models, all with waist-level finders, were produced up to and during World War II. Another ancestor of the modern SLR camera was the Swiss-made Alpa, which was innovative, and influenced the later Japanese cameras. The first eye-level LSR viewfinder was patented in Hungary on August 23, 1943 by Jenő Dulovits, who then designed the first 35mm camera with one, the Duflex, which used a system of mirrors to provide a laterally correct, upright image in the eye-level viewfinder. The Duflex, which went into serial production in 1948, was also the world's first SLR with an instant-return (a.k.a. autoreturn) mirror.
The first commercially produced SLR that employed a roof pentaprism was the East German Contax S, announced on May 20, 1949.

The Japanese adopted and further developed the SLR. In 1952, Asahi developed the Asahiflex and in 1954, the Asahiflex IIB. In 1957, the Asahi Pentax combined the fixed pentaprism and the right-hand thumb wind lever. Nikon, Canon and Yashica introduced their first SLRs in 1959 (the F, Canonflex, and Pentamatic, respectively).

Through The Lens Light Metering (also known as "behind-the-lens metering"

The first manufacturer to produce the first prototype behind-the-lens metering camera was Pentax, which showed their 35mm SLR, the Pentax Spotmatic at a Photokina show circa 1960-1961. Later, Through-the-lens (TTL) light metering SLRs were introduced to the market in the early 1960s, starting with the 1963 production model Topcon RE Super (spot metering) and then later in 1964, a production model of the Pentax Spotmatic (center-weighted average metering) was shown.

Semi-Automatic and Fully-Automatic Exposure Capabilities

While auto-exposure was commonly used in the early 1960's with various 35mm fixed lens rangefinder cameras such as the Konica Auto 'S', and other cameras such as the Polaroid Land cameras whose early models used selenium cell meters, auto-exposure for interchangeable lens SLR's was a feature that was largely absent, except for a few early leaf-shutter SLR's such as the Kowa SE-R and Topcon Auto 100.
The types of automation found in some of these cameras consisted of the simple programmed shutter, whereby the camera's metering system would select a mechanically-set series of apertures with shutter speeds, one setting of which would be sufficient for the correct exposure. In the case of the above-mentioned Kowa and Topcon, automation was semi-automatic, where the camera's CDs meter would select the correct aperture only.
Autoexposure, technically known as semi-automatic exposure, where the camera's metering system chooses either the shutter speed or the aperture, was finally introduced by the Savoyflex and popularized by Konishiroku in the 1965 Konica Auto-Reflex. This camera was of the 'shutter-priority' type automation, which meant that the camera selected the correct aperture automatically. This model also had the interesting ability to photograph in 35mm full-frames or half-frames, all selected by a lever.
Other SLR's soon followed, but because of limitations with their lens mounts, the manufacturers of these cameras had to choose 'aperture-priority' automation, where the camera's metering system selects the correct shutter speed. As one example, Pentax introduced the Electro Spotmatic, which was able to use the then considerable bulk of 42mm screw-mount lenses produced by various manufacturers. Yashica, another screw-mount camera manufacturer, soon followed.
Canon, which produced the FD lens mount (known as the breech-mount; a unique lens mounting system that combines the advantages of screw-mount and bayonet-mount) introduced their shutter priority 35mm SLR, the Canon EF in 1976 or so. This camera's build quality was almost the equal of their flagship camera, the Canon F1, and featured a copal-square vertically-travelling focal plane shutter which could synchronize electronic flash at shutter speeds up to and including 1/125 of a second, thus making this a good second-body camera for the professional photographer.
Nikon at first, produced an aperture-priority camera, but later made subtle changes on the inside of their bayonet mount and soon, shutter-priority automation was achieved.
Full-program auto-exposure soon followed with the advent of the Minolta XD-11, which had a 'P' mode on the shutter speed dial, and a lock on the aperture ring to put the lens on 'Auto'. Other manufacturers soon followed with Nikon introducing the FA, Canon introducing the A1, and Pentax introducing the Super Program. Olympus, however, continued with 'aperture-priority' automation in their OM system line.
The 1970s and 1980s saw steadily increasing use of electronics, automation, and miniaturization, including integrated motor driven film advance with the Konica FS-1 in 1979, and motor rewind functions.

Autofocus

The first phase detection SLR TTL autofocus 35mm SLR was 1981's Pentax ME-F.^[2] The Minolta Maxxum 7000, released in 1985, was the first 35mm SLR with integrated motorized autofocus and film-advance winder, which became the standard configuration for SLR cameras from then on. This development had significant impact on the photographic industry.
Some manufacturers discarded their existing lens systems to compete with autofocus ability in their new cameras, as was the case for Canon with its new EOS lens line. Other manufacturers chose to adapt their existing lens systems for autofocus capability, as was the case with Nikon and Pentax. Still some manufacturers, notably Leica with its R-system lenses, and Contax with its Zeiss lenses, decided to keep their lens mounts non-autofocus. Before the Contax camera and lens line was totally discontinued, Contax did come out with autofocus and digital camera capability but unfortunately it was too late and too expensive for competition with other camera manufacturers. The Contax N-digital was the last Contax to use that maker's lens system, and the camera, while having impressive features such as a full-frame sensor, lacked sufficient write-speed to the memory card for it to be seriously considered by some professional photographers.
From the late 1980s competition and technical innovations made 35mm camera systems more versatile and sophisticated by adding more advanced light metering capabilities such as spot-metering; limited area metering such as used by Canon with the F1 series; matrix metering as used by Nikon, exposure communication with dedicated electronic flash units. The user interface also changed on many cameras, replacing meter needle displays which were galvanometer-based and thereby fragile, with light-emitting diodes (LEDs) and then with more comprehensive liquid crystal displays (LCDs) both in the SLR viewfinder and externally on the cameras' top plate using an LCD screen. Wheels and buttons replaced the shutter dial on the camera and the aperture ring on the lens on many models, although some photographers still prefer shutter dials and aperture rings. Some manufacturers introduced image stabilization on certain lenses to combat camera shake and to allow longer hand-held exposures without using a tripod. This feature is especially useful with long telephoto lenses.

Digital SLRs

Main articles: Digital single-lens reflex camera

Canon, Konica/Minolta, Nikon, Pentax, Samsung, Panasonic and Sony have developed digital SLR cameras compatible with their film SLR systems (though Konica-Minolta recently sold its SLR camera division to Sony, who will continue manufacturing digital cameras using the Maxxum lens mount), while Olympus introduced the new digital-only Four Thirds SLR system, adopted later by Panasonic and Leica.

Format

SLR cameras have been produced for most film formats as well as for digital formats. Most film SLRs use the 35 mm format, as this offers a good compromise of 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 APS such as the Canon IXUS and the Nikon Pronea cameras, but these cameras became less popular with the advent of digital photography. 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 referred to as 'dedicated electronic flash'. In a dedicated system, once the dedicated electronic flash is inserted into the camera's hot shoe and turned on, there is then communication between camera and flash. The camera's synchronization speed is set, along with the aperture. Many camera models on the market today actually measure the light that reflects off of the film plane, controls the flash duration of the electronic flash (some flash units have a flash duration range of 1/1000 of a second to 1/50,000 of a second), and then terminate exposure when the camera has received enough light for the exposure.
Some electronic flash units can send out several short bursts of flash to measure the distance, use the camera's inboard sensor(s) to determine the amount of light that is reflected from the subject, then send out a main pulse of light of just the right amount of light energy for a perfectly exposed photograph. Sophisticated cameras can even make it easy for the photographer to balance electronic flash and available light for an evenly balanced scene. 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 and focusing the image through the attached lens. Most other types of cameras do not have this function; subjects are seen through a viewfinder that is near the lens, making the photographer's view different from that of the lens. SLR cameras provide photographers with precision and confidence; they provide a viewing image that will be exposed onto the negative exactly as it is seen through the lens. There is no parallax error, and exact focus can be confirmed by eye — especially in macro photography and when photographing using long telephoto lenses. The depth of field may be seen by stopping down to the attached lens aperture, which is only possible on most SLR cameras except for the least expensive models. Because of the SLR's versatility, most manufacturers have a vast range of lenses and accessories available for them.
Compared to most fixed-lens compact cameras, the most commonly used and inexpensive SLR lenses offer a wider aperture range and larger maximum aperture (typically to 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. 'Fast' lenses are commonly used in theater photography, portrait photography, surveillance photography, and all other photography requiring a large maximum aperture.
The variety of lenses also allows for the camera to be used and adapted in many different situations. This gives the photographer considerably more control over how the picture is framed than a simple view camera. In addition, SLR lenses can also be found with extremely long focal lengths, letting a photographer be a considerable distance from the subject yet still expose a usable image. This is particularly useful if the subject is dangerous (e.g., wildlife), or the subject prefers anonymity or also if the photographer's presence is unwanted (e.g., celebrity photography or surveillance photography). Practically all SLR camera bodies can also be attached to telescopes and microscopes via an adapter to further enhance their capabilities.

Disadvantages

In most cases, single-lens reflex cameras cannot be made as small or as light as other camera designs — such as rangefinder cameras, autofocus compact cameras and digital cameras with electronic viewfinders (EVF) — owing to the mirror box and pentaprism/pentamirror. The mirror box also prevents lenses from having rear elements closer to the film or sensor to be mounted unless the camera has a mirror lockup feature; this means that simple designs for wide angle lenses cannot be used. Instead, larger and more complex retrofocus designs are required.
The SLR mirror blacks out the viewfinder during exposure. In addition, moving the mirror takes time, limiting the maximum shooting speed; the mirror also causes noise and vibration. Partially-reflective (pellicle) fixed mirrors avoid these problems and have been used in a very few designs including the Canon Pellix, but these reduce the amount of light travelling to the film plane or sensor and also can distort the light passing through them, resulting in a less-sharp image. To avoid the noise and vibration, many professional cameras offer a mirror lock-up feature, but this feature totally disables the SLR's focusing ability.
Most digital SLRs in general cannot display a live view on their rear LCD displays, unlike compact or bridge cameras, and must be held to the eye to compose the picture. This situation is changing with the advent of the Olympus E-330, Olympus E-410, Olympus E-510, the Panasonic DMC-L1, Leica Digilux 3, Canon EOS 40D and the Canon EOS-1D Mark III), and the newly released Nikon D300.
Movie modes are also unavailable.^[3] Electronic viewfinders have the potential to give the advantage of a digital SLR (through-the-lens viewing) without many of the disadvantages, but as of 2006 sensor and display technology is insufficient for wide acceptance among the advanced amateur or professional markets that purchase and use digital SLRs.
The price of SLRs in general also tends to be somewhat higher than that of other types of cameras, owing to the internal complexity. This is only aggravated by the expense of additional components, such as a flash attachment or various types of lenses. Typically the initial investment in equipment is prohibitive enough to keep some casual photographers away from SLRs.
In some cases of particular SLR models, there tends to be a higher rate of breakdowns than a simpler camera of the same build quality. More expensive SLR's, however, tend to be built to much higher standard than other camera types making their reliability better. Because many SLRs have interchangeable lenses, there is a tendency for dust, sand and dirt to get into the main body of the camera through the mirror box when the lens is removed, thus dirtying or even jamming the mirror movement or the shutter mechanism itself. In addition, these particles can also jam or otherwise hinder the focusing feature of a lens. This problem has been somewhat reduced in DSLRs as some cameras have a built-in sensor cleaning unit.

See also

★

★ Box camera

★ Digital SLR

★ Optics

★ Rangefinder camera

★ Scheimpflug principle

★ Twin-lens reflex camera

★ Zeiss Ikon

References

1. One was patented in 1861 but it is not clear if a second example was ever produced; Calvin Rae Smith's design of a Patent Monocular Duplex camera was advertised and sold. Spira, ''The History of Photography,'' 119.
2. History of Innovations 1980–1989 Pentax Imaging Company
3. SLR vs All-in-one: Which way to go? Shawn Barnett

External links and sources

★ Spira, S. F. ''The History of Photography as Seen through the Spira Collection.'' New York: Aperture, 2001. ISBN 0-89381-953-0.

★ Photography in Malaysia's Contax History, Part II.

★ Digital Lens Multiplier Effect Calculator Calculate the Field of View of a 35mm lens when used on a digital SLR

★ SLR Today

★ Photo.net website

★ Leaf shutter designs