Single-lens reflex camera
In photography, a single-lens reflex camera is a type of camera that uses a mirror and prism system to allow photographers to view through the lens and see exactly what will be captured. SLRs became the dominant design for professional and consumer-level cameras throughout the late 20th century, offering interchangeable lenses, through-the-lens metering, and precise framing. Originating in the 1930s and popularized in the 1960s and 70s, SLR technology played a crucial role in the evolution of modern photography. Although digital single-lens reflex cameras succeeded film-based models, the rise of mirrorless cameras in the 2010s has led to a decline in SLR use and production. With twin lens reflex and rangefinder cameras, the viewed image could be significantly different from the final image. When the shutter button is pressed on most SLRs, the mirror flips out of the light path and allows light to pass through to the light receptor and the image to be captured.
History
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 or to the side. 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.Most SLR cameras permit upright and laterally correct viewing through use of a 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 is released, the mirror moves out of the light path, and the light shines directly onto the film. Exceptions to the moving mirror system include the Canon Pellix and Sony SLT cameras, along with several special-purpose high-speed cameras, whose mirror was a fixed beamsplitting pellicle.
Focus can be adjusted manually by the photographer or automatically by an autofocus system. The viewfinder can include a matte 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 cameras 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. SLRs for smaller exposure formats were launched in the 1920s by several camera makers. The first 35 mm SLR available to the mass market, Leica's PLOOT reflex housing along with a 200 mm f4.5 lens paired to a 35 mm rangefinder camera body, debuted in 1935. The Soviet Спорт, also a 24 mm by 36 mm image size, was prototyped in 1934 and went to market in 1937. K. Nüchterlein's Kine Exakta was the first integrated 35 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, 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 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 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 Zeiss Ikon VEB Contax S, announced on May 20, 1949, produced from September.
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.
Digital SLRs
Canon, Nikon and Pentax have all developed digital SLR cameras using the same lens mounts as on their respective film SLR cameras. Konica Minolta did the same, and after having bought Konica Minolta's camera division in 2006 Sony continued using the Minolta AF lens mount in their DSLRs, including cameras built around a semi-transparent fixed mirror. Samsung built DSLRs based on the Pentax lens mount. Olympus, on the other hand, chose to create a new digital-only Four Thirds System SLR standard, adopted later by Panasonic and Leica.Contax released a DSLR model, the Contax N-Digital. This model was too late and too expensive to be competitive 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, was expensive and lacked sufficient write-speed to the memory card for it to be seriously considered by some professional photographers.
The digital single-lens reflex camera largely replaced the film SLR design in convenience, sales and popularity at the start of the 21st century, before the single-lens reflex design in general was supplanted by the mirrorless camera.
Optical components
A cross-section of the optical components of a typical SLR camera shows how the light passes through the lens assembly, is reflected by the mirror placed at a 45-degree angle, and is projected on the matte focusing screen. Via a condensing lens and internal reflections in the roof pentaprism the image appears in the eyepiece. When an image is taken, the mirror moves upwards from its resting position in the direction of the arrow, the focal plane shutter opens, and the image is projected onto the film or sensor 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.
Most 35 mm SLRs use a roof pentaprism or penta-mirror to direct the light to the eyepiece, first used on the 1948 Duflex constructed by Jenő Dulovits and patented August 1943. With this camera also appeared the first instant-return mirror.
The first Japanese pentaprism SLR was the 1955 Miranda T, followed by the Asahi Pentax, Minolta SR-2, Zunow, Nikon F and the Yashica Pentamatic. Some SLRs offered removable pentaprisms with optional viewfinder capabilities, such as the waist-level finder, the interchangeable sports finders used on the Canon F1 and F1n; the Nikon F, F2, F3, F4 and F5; and the Pentax LX.
Another prism design was the porro prism system used in the Olympus Pen F, the Pen FT, the Pen FV half-frame 35 mm SLR cameras. This was later used on the Olympus EVOLT E-3x0 series, the Leica Digilux 3 and the Panasonic DMC-L1.
A right-angle finder is available that slips onto the eyepiece of most SLRs and D-SLRs and allows viewing through a waist-level viewfinder. There is also a finder that provides EVF remote capability.
Shutter mechanisms
Almost all contemporary SLRs use a focal-plane shutter located in front of the film plane, which prevents the light from reaching the film even if the lens is removed, except when the shutter is actually released during the exposure. There are various designs for focal plane shutters. Early focal-plane shutters designed from the 1930s onwards usually consisted of two curtains that travelled horizontally across the film gate: an opening shutter curtain followed by a closing shutter curtain. During fast shutter speeds, the focal-plane shutter would form a 'slit' whereby the second shutter curtain was closely following the first opening shutter curtain to produce a narrow, vertical opening, with the shutter slit moving horizontally. The slit would get narrower as shutter speeds were increased. Initially these shutters were made from a cloth material, but some manufacturers used other materials instead. Nippon Kōgaku, for example, used titanium foil shutters for several of their flagship SLR cameras, including the Nikon F, F2, and F3.Other focal-plane shutter designs, such as the Copal Square, travelled vertically — the shorter travelling distance of 24 millimetres meant that minimum exposure and flash synchronisation times could be reduced. These shutters are usually manufactured from metal, and use the same moving-slit principle as horizontally travelling shutters. They differ, though, in usually being formed of several slats or blades, rather than single curtains as with horizontal designs, as there is rarely enough room above and below the frame for a one-piece shutter. Vertical shutters became very common in the 1980s.
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. An example of a behind-the-lens leaf shutter is found in the 35 mm SLRs produced by Kodak, with their Retina Reflex camera line; Topcon, with their Auto 100; and Kowa with their SE-R and SET-R reflexes. A primary example of a medium-format SLR with a between-the-lens leaf shutter system would be Hasselblad, with their 500C, 500 cm, 500 EL-M and other models. Hasselblads use an auxiliary shutter blind situated behind the lens mount and the mirror system to prevent the fogging of film. Other medium-format SLRs also using leaf shutters include the now discontinued Zenza-Bronica camera system lines such as the Bronica ETRs, the ETRs'i, the SQ and the SQ-AI, and the Zenza-Bronica G system. Certain Mamiya medium-format SLRs, discontinued camera systems such as the Kowa 6 and a few other camera models also used between-the-lens leaf shutters in their lens systems. Thus, any time a photographer purchased one of these lenses, 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 of a second or faster, cameras using leaf shutters were more desirable to studio photographers who used sophisticated studio electronic flash systems. Some manufacturers of medium-format 120 film 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 which was a focal-plane shutter SLR. Rollei later switched to a camera system of leaf-shutter design and their current medium-format SLRs are now all of the between-the-lens shutter design.