Digital single-lens reflex camera
A digital single-lens reflex camera (digital SLR or DSLR) is a digital camera that operates on the same optical and mechanical principles as a modern electronic autofocus 35mm film single-lens reflex camera. The key difference is that the film is replaced with a CCD or CMOS image sensor plus accompanying electronics, thus creating images digitally in-camera, without the need to first chemically develop a latent image on film.
Cross-section view of SLR system.
The single-lens reflex (SLR) camera uses a mirror to show the image that will be captured in a viewfinder. 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 (. 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 sensor (4) in exactly the same manner as on the focusing screen.
Parallax-free optical viewfinder
The principal advantage of DSLR cameras over other digital cameras is the defining characteristic of an SLR: the image in the optical viewfinder is parallax-free, because its light is routed directly from the main lens itself, rather than from an off-axis viewfinder.
However this advantage of seeing the image exactly as it will be captured has been duplicated in many digital compact cameras in their LCD displays. The SLR still retains an advantage because the LCD images are electronically mediated and thus possess time lag, and also have relatively low resolution and brightness making it difficult to see details, especially in outdoor use. In contrast, the SLR's optical image is in real-time and is brighter and more detailed.
Fast phase-detection autofocus
Like most film SLRs, DSLRs typically use a phase detection autofocus system. This method of focus is very fast, and results in less focus "searching", but requires the insertion of a special sensor into the optical path, so it is usually only used in SLR designs. Compact cameras that use the main sensor to create a live preview on the LCD or electronic viewfinder need to use the slower contrast method of autofocus.
The ability to exchange lenses, to select the best lens for the current photographic need, and to allow the attachment of specialized lenses, is a key to the popularity of DSLR cameras, though not inherently unique to DSLRs. The diameters of the lenses for DSLRs are generally larger than for compact point-and-shoots, resulting in more light captured from the subject, allowing the option of using faster shutter speeds, especially in the case of low-light or moving subjects.
In the digital era, zoom lens design is sufficiently advanced to almost eliminate most of the market for non-SLR cameras with interchangeable lenses, so digital camera lens interchangeability is, in practice, mostly restricted to the digital SLR. A non-SLR camera can have a fixed lens which will zoom from medium wide-angle to medium telephoto, omitting only fish-eye and extreme telephoto. However, there are at least four advanced digital non-SLR cameras with interchangeable lenses (see "Compacts" below).
Most lenses intended for film can be used on DSLR cameras, but the reverse is not always true. Newer lenses may use the same style of mount as film cameras, but have newer electronics for image stabilization and image control that a film camera would not recognize. Other lenses designed for DSLR (e.g. Canon EF-S lenses) may not fit on the corresponding film or full-frame bodies; others will fit but their image circle may not cover the full film area.
Sensor size and quality
The Kodak SLR/c: a full-frame DSLR
The image sensor in a DSLR is typically much larger than the one in a consumer-level, compact digital camera. A larger sensor allows better image quality, lower noise, shallower depth of field, higher sensitivity, and increased latitude and dynamic range. Many DSLR sensors are roughly APS-sized, that is, approximately 22 mm x 15 mm, a little smaller than the size of an APS-C film frame, much smaller than a frame of 135 film. A high-end compact camera like the Nikon Coolpix 8800 has an 8.8 by 6.6 mm sensor (2/3 inches format), about 4 to 6 times smaller area than a typical DSLR sensor. Lower end digital compacts have even smaller sensors. Some of the upper-end digital SLRs have sensors the same size as 35mm film, and are referred to as "full-frame" cameras.
The larger size of the sensor of the DSLRs compared to compact digitals makes it much easier to limit the depth of field, for example to emphasize a face by blurring the background so that the viewer will not get distracted by the details in the background. This reduced depth of field can be a disadvantage when the photographer prefers to take pictures where as much as possible is sharply rendered.
Angle of view
The angle of view of a lens depends upon its focal length and the image size; a sensor smaller than 35mm film means that a lens of given focal length will have a narrower angle of view than it would on 35mm film. If the sensor is the same size as the equivalent frame of film (36 mm x 24 mm), the camera is said to have a full-frame sensor. As of 2006, only a few DSLRs have full-frame sensors (currently in production the Canon EOS-1Ds Mark II and the Canon EOS 5D). Medium-format size sensors, as used on the Mamiya ZD and other cameras, are even larger than 35mm or full-frame sensors, and capable of even greater image quality, but are much more expensive.
The impact of sensor size on field of view is referred to as the "crop factor" or "focal length multiplier", which is a factor by which a lens focal length can be multiplied to give the full-frame-equivalent focal length for a lens. Roughly APS-sized sensors have a crop factor of 1.5 or 1.6, so a lens with a focal length of 50mm will give a field of view equal to that of a 75mm or 80mm lens on a full-frame camera. This crop makes achieving long telephoto images on an APS-sensor camera much easier than on a full-frame camera; though wide-angle views suffer by the same amount. Shallow depth-of-field images also tend to be more limited, since the wider the lens you use the more depth-of-field you get, so the smaller the sensor the more depth-of-field with the same aperture and field of view.
Alternatives to the 35mm-based DSLR
Depending on the position of the reflex mirror, the light from the scene can only reach either the viewfinder or the sensor. Therefore, most DSLRs don't provide "live preview" (allowing focusing, framing, and depth-of-field preview using the display). As of February 2007, there are five notable exceptions: the Canon EOS 20Da, Olympus E-330, Panasonic Lumix DMC-L1, and Leica Digilux Three, and the newly introduced Canon EOS-1D Mark III. Additionally, the Fujifilm FinePix S5 Pro offers 30 seconds of "live preview". The upcoming Olympus E-410 (June 2007) and E-510 (July 2007) will offer continuous "live preview". All of these cameras with the exception of the Canon cameras are Four Thirds System cameras.
Many medium format roll-film SLRs can accept a digital camera back to turn the camera into a DSLR with very high image resolution and quality (typically 22-39 megapixels as of January 2007). However, the combination is very expensive and bulky, and more suited to still life than to action photography.
At Photokina in 1986, Nikon revealed a prototype analog electronic still SLR camera, the Nikon SVC, a precursor to the digital SLR . The prototype body shared many features with the N8008.
In 1991 Kodak released the first commercially available digital SLR, the Kodak DCS-100. It consisted of a modified Nikon F3 SLR body, modified drive unit, and an external storage unit connected via cable. The camera was capable of producing 1.3 megapixel (1280x1024) images and cost approximately US$30,000. This was followed by the Kodak DCS-200 with integrated storage.
Over the next decade, digital SLRs have been released by various companies such as Canon, Nikon, Kodak, Pentax, Olympus, Panasonic, Samsung, Konica Minolta, Fujifilm, and Sigma with higher resolution and lower prices.
In January 2000 Fujifilm announced the FinePix S1 Pro which was the first DSLR that had a 'reasonable' price and was marketed to non-professionals.
In 2003 Canon introduced the 6.3 megapixel EOS 300D SLR camera (known in the United States as the Digital Rebel) at an MSRP of under US$1000. Its popularity, especially among newspaper and amateur photographers, encouraged other manufacturers to produce affordable digital SLR cameras, significantly lowering entry costs and allowing more casual photographers an opportunity to experience the digital SLR photography. Canon introduced the next generation 8 megapixel EOS 350D (Digital Rebel XT) in 2005, then followed that with the 10 megapixel EOS 400D (Digital Rebel XTi) in 2006.
In February 2004 Kodak released two 14-megapixel full-frame (24x36mm) DSLRs named Kodak DCS 14 N ,Kodac DSC 14 C, one for Nikon lens mount and one for Canon lens mount. Kodak has since discontinued them. Konica Minolta briefly produced two DSLR models, the 5D and 7D, but has sold their digital camera business to Sony which revamped the range through its α (alpha) brand. At one time, Kyocera also manufactured DSLRs and marketed them under the Contax name, but in 2005 withdrew from the DSLR camera field. Sigma Corporation of Japan produced the Sigma SD9 and Sigma SD10 DSLR cameras with the Foveon X3 sensor, but those cameras are no longer in production; the successor to this line, the Sigma SD14, was announced at Photokina in 2006.
Canon, Nikon, Pentax, Olympus, Sony and Fujifilm currently have DSLR models in production.
Canon and Nikon currently have the largest range of amateur and professional DSLR cameras. Canon's line includes the 400D, 350D, 30D, 5D, 1D Mk. IIn, 1Ds Mk. II, and the Canon EOS-1D Mark III, all of which have CMOS sensors. Nikon also has a broad line of DSLRs which includes the D40, D40x, D50, D70s, D80, D200, D2Hs and D2Xs. Fujifilm sells Nikon-lens compatible DSLR cameras, the FinePix S3 Pro and FinePix S5 Pro. Olympus, together with Panasonic and Leica, make DSLR cameras that conform to the Four Thirds System, including several models that feature a live preview LCD in addition to the viewfinder. Sigma produces an innovative DSLR with the multi-layered Foveon X3 sensor to deliver excellent color and detail, while Pentax also has digital SLRs that use their lenses and accessories. Hasselblad and Mamiya produce medium-format DSLRs which produce the highest quality digital images. Their larger sensors are able to capture much more detail than the 35mm full-frame and smaller sensors found on the Canon, Nikon, Olympus, Pentax, and Sigma models.
Digital SLR versus SLR-like and compact cameras
Non-SLR digital cameras are of two types: SLR-like prosumer digital cameras (also called advanced digital cameras) and compact digital cameras. Both have permanently fixed lenses.
The "SLR-like" or "advanced" cameras (e.g. some Nikon Coolpix models, the Sony DSC V, Sony DSC H and Sony DSC R series, the Panasonic FZ series, the Canon PowerShot A, G, S and Pro models, Minolta DiMAGE series (DiMAGE 7, 7Hi, A1, A2, A200) and several Fujifilm FinePix models) offer through-the-lens (TTL) viewing through the focusing lens, projected onto its viewfinder as well as a LCD screen, through an EVF (electronic viewfinder). The difference compared to a DSLR is that the viewfinder shows a digitally created copy of the TTL image, whereas the viewfinder in a DSLR shows the actual optical TTL image via its mirror. An EVF image reacts more slowly to view changes and has a lower resolution than an optical viewfinder, but achieves parallax-free viewing using less bulk and mechanical complexity than a DSLR with its reflex viewing system. The limitation of a prosumer digital camera is its fixed lens, typically limited to about a 12:1 focal-length range. The fixed lens also minimizes the risk of getting dust on the sensor.
Several of the high-end prosumer cameras have a movable LCD screen (e.g. Canon PowerShot Pro1 and Konica Minolta A200), which can be used instead of the regular viewfinder in difficult angles, enabling the photographer to look through the lens even when standing above or below the camera. The LCD screen also enables the photographer to look at the stored pictures in a convenient way.
High-end prosumer cameras such as the Konica Minolta A200 provide image stabilization via CCD sensor shift; this stabilization can reduce the blurring effect of camera shake.
The compact digital cameras, commonly referred to as 'point-and-shoot' cameras because of their ease of use, can usually be operated at arm's length using only the LCD display screen at the rear of the camera, and most models also have simple optical viewfinders like traditional compact film cameras. Like the SLR-like prosumer cameras, nearly all compacts have no ability to accept interchangeable lenses, with the exception of the Epson R-D1 and the Leica M8.
Most compacts are therefore provided with a zoom lens that covers the most commonly used fields of view. Compacts may be altered through the use of supplementary add-on lens converters to provide an added telephoto or wide angle field of view, though the image quality is usually affected to a significant degree. Most compacts are significantly slower in image capture (time from shutter release to storage) than DSLR cameras, a disadvantage for action, wildlife, and sports photography. Their zoom lenses can frequently have a much slower (smaller) effective wide-open aperture (f-number) than DSLR or prosumer cameras, especially at the telephoto end, which further limits their utility in situations involving low light levels and moving subjects. Image stabilization is a feature available in some compact cameras that can effectively remedy the issue of camera-shake in low-light conditions without moving subjects and therefore reduce blurring in low-light photographs.