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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. more...

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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.

The 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 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 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.

History

Large format SLR cameras were probably first sold in 1884. 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 Jenő Dulovits. The first 35mm camera that had one implemented was the Duflex, designed by Dulovits. This camera 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.

Read more at Wikipedia.org