|
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. 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.
History
Autofocus 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 1980's 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. Digital SLRs 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. Format 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. Common features 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. Advantages 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 to 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 particuarly useful if the subject is dangerous (i.e., wildlife), or the subject would prefer the photographer to stay away (i.e., a celebrity). Disadvantages 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. 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 and related Panasonic DMC-L1). Movie modes are also unavailable •. 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. This is only aggravated by the fact that additional components, such as a flash attatchment or various types of lenses, can be extremely expensive as well. Typically the initial investment in equipment is prohibitive enough to keep the casual photographer away from SLRs. There is also the obvious problem of a higher rate of breaking down due to more moving components. Because many SLRs have interchangeable lenses, there is a tendency for dust, sand and dirt to get into the main body of the camera, dirtying or even jamming the mirror's movement. In addition, these particles can also jam or otherwise hinder the focusing feature of a lens. This problem has been somewhat reduced as some cameras have a built-in sensor cleaning unit. See also | ||||||||||
|
| |||||||||||
 |
|
| |