Editing Single-lens reflex camera (section)

== History ==
{{Main|History of the single-lens reflex camera}}
[[File:Nikon SLR Camera cutaway.jpg|thumb|left|Cross section of SLR camera.]]
Until the development of SLR, all cameras with viewfinders had two optical light paths: one through the lens to the film and another positioned above ([[Twin-lens reflex camera|TLR or ''twin-lens reflex'']]) or to the side ([[Rangefinder camera|rangefinder]]). Because the viewfinder and the film lens cannot share the same optical path, the viewing lens is aimed to intersect with the film lens at a fixed point somewhere in front of the camera. This is not problematic for pictures taken at a middle or longer distance, but [[parallax]] causes framing errors in close-up shots. Moreover, it is not easy to focus the lens of a fast reflex camera when it is opened to wider [[apertures]] (such as in low light or while using low-speed film).

Most SLR cameras permit upright and laterally correct viewing through use of a [[pentaprism|roof pentaprism]] situated in the optical path between the reflex mirror and viewfinder. Light, which comes both horizontally and vertically inverted after passing through the lens, is reflected upwards by the reflex mirror, into the [[pentaprism]] where it is reflected twice to correct the inversions caused by the lens, and align the image with the [[viewfinder]]. When the [[Shutter (photography)|shutter]] is released, the mirror moves out of the light path, and the light shines directly onto the film (or in the case of a [[Digital single-lens reflex camera|DSLR]], the [[Charge-coupled device|CCD]] or [[CMOS]] imaging sensor). Exceptions to the moving mirror system include the Canon Pellix and [[Sony_SLT_camera|Sony SLT cameras]], along with several special-purpose high-speed cameras (such as the Canon EOS-1N RS), whose mirror was a fixed [[Beam splitter|beamsplitting]] [[Pellicle mirror|pellicle]].

Focus can be adjusted manually by the photographer or automatically by an [[autofocus]] system. The viewfinder can include a matte [[Focus (optics)|focusing]] screen located just above the mirror system to diffuse the light. This permits accurate viewing, composing and focusing, especially useful with interchangeable lenses.

Up until the 1990s, SLR was the most advanced photographic preview system available, but the recent development and refinement of digital imaging technology with an on-camera live LCD preview screen has overshadowed SLR's popularity. Nearly all inexpensive compact digital cameras now include an LCD preview screen allowing the photographer to see what the CCD is capturing. However, SLR is still popular in high-end and professional cameras because they are [[system camera]]s with interchangeable parts, allowing customization. They also have far less [[shutter lag]], allowing photographs to be timed more precisely. Also the [[pixel]] resolution, [[contrast ratio]], [[refresh rate]], and color [[gamut]] of an LCD preview screen cannot compete with the clarity and shadow detail of a direct-viewed optical SLR viewfinder.

[[Large format]] SLR cameras were probably first marketed with the introduction of C.R. Smith's ''Monocular Duplex'' (U.S., 1884).<ref>One was patented in 1861 ([[Thomas Sutton (photographer)|Thomas Sutton]]), 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.</ref> SLRs for smaller exposure formats were launched in the 1920s by several camera makers. The first [[35&nbsp;mm format|35&nbsp;mm]] SLR available to the mass market, Leica's PLOOT reflex housing along with a 200&nbsp;mm f4.5 lens paired to a [[35&nbsp;mm format|35&nbsp;mm]] rangefinder camera body, debuted in 1935. The Soviet [[Sport_(camera)|''Спорт'' (“Sport”)]],<ref>[[:ru:Спорт (фотоаппарат)|A. O. Gelgar's ''Sport'']]</ref> also a 24&nbsp;mm by 36&nbsp;mm image size, was prototyped in 1934 and went to market in 1937. K. Nüchterlein's [[Kine Exakta]] (Germany, 1936) was the first integrated [[35&nbsp;mm format|35&nbsp;mm]] SLR to enter the market. Additional [[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,<!-- How was it innovative? --> and influenced the later Japanese cameras. The first eye-level SLR viewfinder was patented in Hungary on August 23, 1943, by Jenő Dulovits, who then designed the first 35&nbsp;mm 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 Italian [[Rectaflex]] A.1000, shown in full working condition at a Milan fair in April 1948 and produced from September the same year. It was thus on the market one year before the East German [[Contax|Zeiss Ikon VEB Contax S]], announced on May 20, 1949, produced from September.

The Japanese adopted and further developed the SLR. In 1952, [[Pentax|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 Corporation|Nikon]], [[Canon Inc.|Canon]] and [[Yashica]] introduced their first SLRs in 1959 (the [[Nikon F|F]], [[Canon Canonflex|Canonflex]], and Pentamatic, respectively).