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.
Cross-section view of SLR systemThe 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.
The 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 onto the film or sensor (4) in exactly the same manner as on the focusing screen.
This feature separates 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).
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 landscape photographers may prefer view cameras
Large format SLR cameras were first built in the early years of the 20th century. 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 proved influential for the later Japanese cameras. The first solution for an eye-level viewfinder was patented in Hungary during the war—more precisely, on August 23, 1943, by Jen? Dulovits. The first 35mm camera that had one implemented was the Duflex, designed by Dulovits. This camera utilised 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 serially produced SLR that employed a roof pentaprism was the East German Contax S, announced on May 20, 1949.
The Japanese further developed the SLR. In 1952, Asahi developed the Asahiflex and in 1954, the Asahiflex IIB. In 1957, Asahi Pentax introduced 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 (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 Program, 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 miniaturization, including integrated motor driven film advance with the Konica FS-1 in 1979, and motor rewind functions.
The first phase detection SIR TTL autofocus SLR was 1981’s Pentax ME-F. The Minolta Maxxum 7000, released in 1985, was the first SLR with integrated motorised autofocus and film advance winder – which became the standard configuration for SLR-cameras from then on. This had significant impact on the industry. Some manufacturers discarded their existing systems to enter the AF era, while others chose to adapt their systems for compatibility.
From the late 1980s competition and technical innovations made the camera systems more “intelligent” by adding more advanced light metering, and by allowing the different components to exchange information electronically. The user interface also changed on many cameras, replacing needle display and LEDs with more comprehensive LCD displays both in viewfinder and externally on the cameras. Wheels and buttons replaced the shutter dial on the camera and the aperture ring on the lens on many models. Some manufacturers also introduced antishake features on some lenses to allow for longer exposure-times without using support.
Canon, Nikon, Samsung, Pentax, and Minolta 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), while Olympus and Panasonic have introduced a new digital-only SLR system, the Four Thirds system.
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.
Many of the advantages of SLR cameras derive from viewing the scene through the taking lens. Most other types of camera do not have this function; subjects are seen through a viewfinder that is near the lens, making the photographer’s view different from the lens’ view. SLR cameras provide photographers with precision and confidence; they are seeing an 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 — 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.
The variety of lenses also allows for the camera to be used in multiple 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 points, letting a photographer be far away from the subject. This is particularly useful if the subject is dangerous (e.g., wildlife), or the subject would prefer the photographer to stay away (e.g., a celebrity).
Single-lens reflex cameras cannot be made as small or light as other camera designs — such as rangefinder cameras, autofocus compact cameras and digital cameras with electronic viewfinders (EVF) — due to the mirror box and pentaprism/pentamirror. The mirror box also prevents lenses from having rear elements closer to the film or sensor; this means that simple designs for wide angle lenses cannot be used. Instead, poorer-performing, larger and more complex retrofocus designs are required.
The SLR mirror blacks out the viewfinder when the picture is taken. In addition, moving the mirror takes time, limiting the maximum shooting speed; the mirror also causes noise and vibration. Some SLRs have used partially-reflective fixed mirrors to avoid these problems, including the Canon Pellix, but these reduce the light getting to the film or sensor. To avoid the noise and vibration, many professional cameras offer a mirror lock-up feature, but this blacks out the viewfinder totally when in use.
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 (with the exception of the Olympus E-330, the Panasonic DMC-L1 and the Leica Digilux 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 buy digital SLRs.
The price of SLRs in general also tends to be much higher than that of other types of cameras, due to the internal complexity of the device.
There is also the obvious problem of a higher rate of breaking down than a simpler camera of the same build quality due to more moving components.