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.The shutter in almost all contemporary SLRs sits just in front of the focal plane.
This 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 in the film (4) in exactly the same manner as on the focusing screen.
Since the technology became widespread in the 1970s, SLRs have become the main type of camera used by dedicated amateur photographers and professionals. Large format SLR cameras were first built in the early years of the 20th century but were not very popular. Nikon, Canon, and Yashica introduced their first SLRs in 1959; the Nikon F was the camera that switched professional photographers to the 35 mm SLR. It was highly modular, extremely well-built and versatile, and began the F series that continues to the present day.
Canon, Nikon, Pentax and Minolta have all introduced commercially successful digital SLR ranges compatible with their film SLR systems (though Konica-Minolta recently sold its SLR camera division to Sony who will continue manufacture), while Olympus has 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 2005 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.
Common features Edit
Other features found on many SLR cameras include through-the-lens (TTL) metering and sophisticated flash control. Many models on the market today actually measure the light that bounces off the film, and close the shutter when the picture has had enough exposure. Likewise, they can send out several short bursts of flash, determine the amount that comes back from the scene, then send out just the right amount of energy for a perfectly exposed photograph. Sophisticated cameras can even make it easy for the photographer to balance flash and available light for the desired look. 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.
Many of the advantages of SLR cameras derive from viewing the scene through the taking 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. Only the Leica rangefinder cameras have a comparable system.
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, which makes the subject stand out better. This is commonly used in portrait photography.
In the diagram the light path through the roof pentaprism is shown in simplified form. In fact an odd number of reflections are required to rectify the image on the ground glass screen. The first reflection from the moving mirror requires an additional three reflections inside the prism. Two only are shown.
The most obvious disadvantage of the SLR is its inability to view the scene at the moment the shutter captures the image. A second disadvantage is a normally greater weight and size than rangefinders of a similar technology level - the pentaprism and mirror box make the camera body larger. However, rangefinders have not advanced significantly since the 1970s, while modern SLRs use advanced automation, plastics, and electronics to be smaller - sometimes at the price of long-term durability and reliability. Plastic moving parts don't last as long as metal.
The SLR's space-consuming mirror movement makes for difficulty in constructing wide angle lenses; rear lens elements cannot be close to the film plane. Retrofocus designs are required for wide-angle lenses; these are complex, large, and comparatively poorer in image quality.
The reflex mirror must retract before the shutter can open, which introduces some delay. Autofocus systems on modern SLRs introduce further delay, especially in lower light. The mirror's movement also causes vibration and noise, a problem when using longer lenses and longer exposures. Technology has reduced but not eliminated this problem, which again is worse in larger formats. To combat this, higher-end cameras offer the ability to lock up the mirror before the shot is taken. This eliminates the vibration but blacks out the viewfinder.
The SLR user cannot see anything outside the taking frame through the viewfinder, while with most rangefinder systems, this can be done. This helps in certain kinds of photography. Only higher-end SLRs show the full frame; typical coverage is 90%. Print labs generally crop an equivalent area, so it is less of a problem than it might otherwise be.