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An example of a modern digital SLR (the Pentax K10D) is pictured without a lens installed. Also shown is the chrome interchangeable lens mount with electronic contacts and the reflex mirror inside of the camera.

A modern DSLR, the Canon EOS 40D, with an 85mm prime lens attached

A digital single-lens reflex camera (digital SLR or DSLR) is a digital camera that uses an automatic mirror system and pentaprism to direct light from the lens through the viewfinder.

The basic operation of a DSLR is as follows: for viewing purposes, the mirror reflects the light coming through the attached lens upwards at an approximately 90 degree angle. It is then reflected by the pentaprism to the photographer's eye. During exposure (when the photograph is taken), the mirror swings upward, allowing the lens to project light onto the image sensor.

This design scheme is a major difference compared to an ordinary digital camera (digicam), which typically exposes the sensor constantly to the light projected by the lens, allowing the camera's screen to be used as an electronic viewfinder.

In contrast, the mirror arrangement in a DSLR usually precludes the ability to view the scene on the LCD display screen before the photograph is taken. However, many newer DSLR models feature live preview, allowing LCD display to be used as a viewfinder in the same way as a normal digicam, although with certain limitations and with the optical viewfinder disabled.

In most other respects, a DSLR is similar in principle and operation to a standard (non-SLR) digital camera.

Cross-section view of SLR system.
1 - 4-element lens
2 - Reflex mirror
3 - Focal-plane shutter
4 - Sensor
5 - Matte focusing screen
6 - Condenser lens
7 - Pentaprism
8 - Eyepiece

A camera based on the single-lens reflex (SLR) principle 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 into the pentaprism by the reflex mirror (which must be at an exact 45 degree angle) (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 is projected, either focused or unfocused through the eyepiece (8) to the photographer's eye. Focusing is usually automatic, activated by pressing half-way on the shutter release or a dedicated AF button, as is mainly the case with an autofocusing film SLR; or manual, where the photographer manually focuses the lens by turning a lens ring on the lens barrel. When an image is photographed, the mirror moves upwards in the direction of the arrow, the focal-plane shutter (3) opens, and the image is projected and captured on the sensor (4), after which actions, the shutter closes, the mirror returns to a critical 45 degree angle, and the diaphragm reopens and the built in drive mechanism retensions the shutter for the next exposure.

The diagram shown here is an over-simplification in that it omits the sensors used to activate the drive for the autofocus system.

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. Digicams 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.

Depending on the viewing position of the reflex mirror (down or up), the light from the scene can only reach either the viewfinder or the sensor. Therefore, most DSLRs do not currently provide "live preview" (allowing focusing, framing, and depth-of-field preview using the display), a facility that is always available on digicams.

The advantages of an optical viewfinder are that it alleviates eye-strain sometimes caused by electronic view finders (EVF), and that it constantly shows (except during the split second of time for the sensor to be exposed) the exact image that will be exposed because its light is routed directly from the lens itself. Compared to ordinary digital cameras with their LCD displays and/or electronic viewfinders the advantage is that there is no time lag in the image; it is always correct as it is being "updated" at the speed of light. This is important for action and/or sports photography, or any other situation where the subject or the camera is moving too quickly. Furthermore, the "resolution" of the viewed image is much better than that provided by an LCD display or an electronic viewfinder, which can be important if manual focusing is desired for precise focusing, as would be the case in photomacrography or photomicrography (commonly referred to as "macro-photography" and "micro-photography").

Compared to some low cost cameras that provide an optical viewfinder that uses a small auxiliary lens, the DSLR design has the advantage of being parallax-free; that is, it never provides an off-axis view.

A disadvantage of the DSLR optical viewfinder system is that it prevents the possibility of using the LCD display for viewing and composing the picture before taking it. Some people prefer to compose pictures on the display – for them this has become the natural way to use a camera. LCD displays and electronic viewfinders may also provide a brighter display in low light situations, as the picture can be electronically amplified; conversely, LCD displays can be difficult to see in very bright sunlight.

A fairly recent development in DSLRs is the emergence of live preview options, which make it possible to use either the optical viewfinder or the LCD display when composing the picture (but not both). This can be an advantage because some people simply prefer to use the display and because in some situations it is not convenient or possible to hold the camera up to one's face to look through the viewfinder. Underwater photography, where the camera is enclosed in a plastic waterproof case, is an example of a situation where composing on the display is preferred.

Olympus introduced the first DSLR with live preview - albeit an atypical design with a fixed lens - the Olympus E-10, in the summer of 2000. Since then other manufacturers have launched DSLR models with live preview, and the possibility exists that eventually all new DSLRs will have this feature.

As of late 2007, the Canon EOS-1D Mark III, Canon EOS-1Ds Mark III, Canon EOS 40D, Nikon D3, Nikon D300, Olympus E-330, Olympus E-410, Olympus E-510, Olympus E-3, Panasonic Lumix DMC-L1, Panasonic Lumix DMC-L10 and Leica Digilux 3 all provide continuous live preview as an option. Additionally, the Fujifilm FinePix S5 Pro^[1] offers 30 seconds of live preview.

Some live preview systems make use of the primary sensor to provide the image on the LCD display (which is the way all non-DSLR digicams work), and some systems use a secondary sensor. Possible advantages of using a secondary sensor for live preview is to avoid additional noise that might result from the primary sensor heating up from continuous use, and allowing faster auto-focus.^[2]

A new feature via a separate software package introduced from Breeze Systems in October, 2007, features live view from a distance. The software package is named "DSLR Remote Pro v1.5" and enables support for the Canon EOS 40D and 1D Mark III.^[3]

Main articles: Photographic lens and Lenses for SLR and DSLR cameras

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.

Nikon D200 digital SLR with a 12-24mm Nikkor zoom lens

Interchangeable lenses for SLRs and DSLRs are built to operate correctly with a specific lens mount. A photographer will often use lenses made by the same manufacturer as the camera body (for example, Canon lenses on a Canon body). There are also many independent lens manufacturers, such as Sigma,^[4] Tamron,^[5] Tokina,^[6] and Vivitar,^[7] to name a few, that make lenses for a variety of different lens mounts. There are also lens adapters that allow a lens for one lens mount to be used on a camera body with a different lens mount, but with reduced functionality.

Many lenses are mountable, "diaphragm-and-meter-compatible," on modern DSLRs and on older film SLRs that use the same lens mount. For more information see Mount compatibility across camera generations.

Some DSLR's use a pentamirror instead of the traditional pentaprism. The pentamirror design is composed mostly of plastic and is lighter and cheaper to produce.

Drawing showing the relative sizes of sensors used in current DSLRs.

Image sensors used in DSLRs come in a range of sizes. The very largest are the ones used in "medium format" cameras, typically via a "digital back" which can be used as an alternative to a film back. Because of the manufacturing costs of these large sensors the price of these cameras is typically over $20,000 as of December 2007.

Neglecting medium format DSLRs, the largest sensors are referred to as "full-frame", and are the same size as 35 mm film (135 film); these sensors are used in quite expensive DSLRs such as the Canon EOS-1Ds Mark III, the Canon EOS 5D, and the Nikon D3. Most modern DSLRs use a smaller sensor commonly referred to as APS-C sized, that is, approximately 22 mm × 15 mm, a little smaller than the size of an APS-C film frame, or about 40% of the area of a full-frame sensor. Other sensor sizes found in DSLRs include the Four Thirds System sensor at 26% of full frame, APS-H sensors (used, for example, in the Canon EOS-1D Mark III) at around 61% of full frame, and the Foveon X3 sensor at 33% of full frame.

All of the sensors used in current DSLRs are much larger than the sensors found in digicam-style cameras, most of which use sensors known as 1/2.5", whose area is only 3% of a full frame sensor. Even high-end digicams such as the Canon PowerShot G9 or the Nikon CoolPix P5000 use sensors that are approximately 5% and 4% of the area of a full frame sensor, respectively. For more information see image sensor format.

There is a connection between sensor size and image quality; in general, a larger sensor provides lower noise, higher sensitivity, and increased latitude and dynamic range. There is also a connection between sensor size and depth of field, with the larger sensor resulting in shallower depth of field.

The larger size of the sensors (and therefore lens focal lengths) of DSLRs compared to digicams makes it much easier to limit the depth of field, for example to emphasize a face by blurring the background. This reduced depth of field can be a disadvantage when the photographer prefers to take pictures where as much of the scene as possible is sharply rendered.

Note that DSLR's typically have lens apertures smaller than the typical digicam's aperture setting ability, which is usually f/8. This is a limitation of the small sensor. In contrast, a DSLR can 'stop down' to f/16, f/22 or smaller aperture, depending upon the lens mounted on the camera and its f/stop range.

The angle of view of a lens depends upon its focal length and the camera's image sensor size; a sensor smaller than 35mm film format (36mm × 24mm frame) gives a narrower angle of view for a lens of a given focal length than does a camera equipped with a full-frame (35mm) sensor. As of 2007, only a few current DSLRs have full-frame sensors, including the Canon EOS-1Ds Mark II, the Canon EOS 5D, and the Nikon D3. The scarcity of full-frame DSLRs is partly a result of the cost of such large sensors. Medium format size sensors, such as those used in the Mamiya ZD among others, are even larger than full-frame (35mm) sensors, and capable of even greater image quality, but are even 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 to 1.7, so a lens with a focal length of 50mm will give a field of view equal to that of a 75mm to 85mm lens on a full-frame camera. This crop factor makes achieving long telephoto images on an APS-sensor camera 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 f-number and field of view.

Digital SLR cameras, along with most other digital cameras, generally have a mode dial to access standard camera settings or automatic scene-mode settings. Sometimes called a "PASM" dial, they typically provide as minimum Program, Aperture-priority, Shutter-priority, and full Manual modes. Scene modes vary and are inherently less customizable. They often include full-auto, landscape, portrait, action, macro, and night modes, among others. Professional DSLRs seldom contain automatic scene modes because professionals understand their equipment and can quickly adjust the settings to take the image that they want.

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.

As of 2007 integrated medium formats like the Hasselblad H System^[8] and Leaf AFi^[9] have started to appear.

On July 13, 2007, FujiFilm announced the FinePix IS Pro, which uses Nikon F-mount lenses. This camera, in addition to having live preview, has the ability to record in the infrared and ultraviolet spectrums of light.^[10]

At Photokina in 1986, Nikon revealed a prototype analog electronic still SLR camera, the Nikon SVC, a precursor to the digital SLR ^[11]. The prototype body shared many features with the N8008.^[11]

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 1.3 megapixel camera cost approximately US$30,000. This was followed by the Kodak DCS-200 with integrated storage.^[12]

Over the next decade, DSLRs have been released by various companies such as Canon, Nikon, Kodak, Pentax, Olympus, Panasonic, Samsung, Konica Minolta, which was acquired by SONY, Fujifilm, and Sigma with higher resolution and lower prices.

In 1999, Nikon announced the Nikon D1, the first DSLR to truly compete, and begin to replace, film cameras in the professional photojournalism and sports photography fields. This camera was able to use current autofocus Nikkor lenses available at that time for the Nikon film series cameras, and was also able to utilize the older Nikon and similar, independent mount lenses designed for those cameras. A combination of price, speed, and image quality was the beginning of the end of 35mm film for these markets.

In January 2000, Fujifilm announced the FinePix S1 Pro, the first DSLR 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) with an MSRP of US$999. Its popularity encouraged other manufacturers to produce affordable digital SLR cameras, lowering entry costs and allowing more amateur photographers to purchase DSLR's.

Since 2003, the number of megapixels in imaging sensors have increased steadily, with most companies focusing on build quality, high ISO performance, speed, the elimination of digital 'noise' produced by the imaging sensor, and price reductions to lure new customers.

Canon, Fujifilm, Nikon, Olympus, Panasonic, Pentax, Sigma, and Sony currently produce DSLR cameras. Hasselblad and Mamiya also produce expensive, professional medium-format DSLRs.

• Canon's EOS digital line includes the 400D, 40D, 5D and the 1D Mark III. Canon's latest cameras, the 40D and the 1Ds Mark III were introduced on August 20, 2007. All current Canon DSLRs use CMOS sensors.

• Nikon also has a broad line of DSLRs currently including the D40, D40x, D80, D200, D2Hs, and D2Xs. Nikon announced two new cameras on August 23, 2007, the D300^[13] and the D3. The latter is the company's first full-frame digital SLR.

• Fujifilm currently sells the Fujifilm FinePix S5 Pro DSLR camera body, compatible only with the Nikon F-mount lens system. This camera is based upon the magnesium-alloy body of the Nikon D200, but utilizes Fuji's sensor (Fujifilm SuperCCD SR Pro) technology and Fuji's menu system. Another DSLR from Fuji is the Fujifilm FinePix IS Pro, which has the unique ability to capture light in the infrared and ultraviolet spectrums.

• Olympus, Panasonic, and Leica make DSLR cameras that conform to the Four Thirds System of bodies and lenses designed specifically for digital photography. Current Olympus models are the E-410, E-510, and E-3, while the Lumix DMC-L10 is the latest Panasonic model. Innovative features of these models include live preview in addition to the optical viewfinder, and supersonic dust reduction. The Olympus E-3 and E-510 feature in-body image stabilization while Panasonic and Leica feature in-lens stabilization.

• Pentax (in collaboration with Samsung) currently offers the Pentax K100D Super and K10D. The Samsung GX-10 is a clone of the K10D. Innovative features include in-body image stabilization, dust reduction system, use of standard AA batteries in the K100D Super, weather-proof sealing on the K10D (a feature otherwise found only in semi-pro models such as the Nikon D200), and adoption of Adobe's DNG standard raw image format. Also, these cameras are based on the Pentax K mount lens system, so they can accept all K-mount lenses made since 1975 (though the automatic light metering functionality of some early lenses does not work).

• Sony, which acquired Konica Minolta's DSLR line in 2006, produces the Sony α line of DSLRs with in-body image stabilization.

• Sigma produces an innovative^{[citation needed]} DSLR with the multi-layered Foveon X3 sensor to deliver excellent^{[citation needed]} color and detail.^{[citation needed]}

• Hasselblad and Mamiya produce medium format-sized DSLRs which produce the highest quality digital images. Their high-resolution sensors (over 39 megapixels in some cases) are able to capture much more detail than the 35 mm full-frame and smaller sensors found in other cameras.

Non-SLR digital cameras generally fall into two types: compact digicams, and SLR-like bridge digital cameras (also known as advanced digital cameras) which offer larger zoom ranges, better optics, and more manual controls. Both types have permanently fixed lenses. While the only defining feature of an SLR is its reflex viewfinder system, extant digital SLR models generally offer the following advantages over fixed-lens cameras of the same generation:

• Choice of interchangeable^[14] (and often higher-quality) lenses.
• Image sensors of much larger size and often higher quality, offering lower noise^[15], which is useful in low light, and greater dynamic range.^[16]
• Optical viewfinders which tend to be more comfortable and efficient, especially for action photography and in low-light conditions.
• DSLRs often offer faster and more responsive performance, with less shutter lag, faster autofocus systems, and faster frame rates. ^[17]
• The larger focal length for the same field of view allows creative use of depth of field effects. ^[18]
• Ability to attach additional accessories including hot shoe-mounted flash units, battery grips for additional power and hand positions, external light meters, and remote controls

There are also certain drawbacks to current DSLR designs, compared to fixed-lens cameras:

• Generally greater size and weight. ^[19]
• Generally greater cost. ^[20]
• Lack of a video mode.^[21] No DSLRs produced so far can record full-motion video, while this has become a standard feature of compact digital cameras.
• Many DSLR models lack live preview on their LCD screens, a universal feature among compact digital cameras.
• Noisier operation, due to the SLR mirror mechanism.^[22]
• Potential contamination of the sensor by dust particles, when the lens is changed (though recent dust reduction systems alleviate this).
• Small digicams generally can focus on closer objects than typical DSLR lenses.^[23]

The "SLR-like" or "advanced" digicams (e.g. the Olympus SP series, Nikon Coolpix models such as the 5700, 8400, 8700 and 8800 to name a few; the Sony Cyber-shot DSC V, DSC H, DSC F and DSC R series, the Panasonic FZ series, the Canon PowerShot A, G, some S and Pro models, Minolta DiMAGE 7/7i/7hi, A1, A2, A200, and some Fujifilm FinePix models such as the S7000 and S9000) offer a non-optical electronic through-the-lens (TTL) view through the focusing lens, via the eye-level electronic viewfinder (EVF) as well as the rear LCD screen.

The difference in views compared to a DSLR is that the EVF shows a digitally-created TTL image, whereas the viewfinder in a DSLR shows an actual optical TTL image via the reflex viewing system. An EVF image has lag-time (i.e., it 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.

One limitation of a bridge digital camera is its fixed, non-interchangeable lens. Some of these lenses are limited to approximately a 7:1 zoom ratio range, as is the range of the Fujifilm FinePix S9100; to about a 12:1 focal-length range with the Canon PowerShot S5-IS. An advantage, however, is that the fixed zoom lens eliminates the risk of dust accumulating on the imaging sensor from the changing of an interchangeable lens.

Several of the high-end bridge cameras such as the current Canon PowerShot S5-IS have a movable LCD screen (other examples: the now-discontinued Canon PowerShot Pro1 and Konica Minolta A200), which can be used instead of the regular viewfinder to provide waist-level or low-angle viewing, overhead viewing, and viewing for a self-timer group shot or portrait. The LCD screen also enables the photographer to review the stored images in a convenient way.

Digicams, some 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. Some models also have simple optical viewfinders like traditional compact 35mm film cameras. Like the SLR-like bridge cameras, digicams lack the ability to accept interchangeable lenses, with the exception of certain digital rangefinder cameras such as the Leica M8 and the Epson RD-1, which use the Leica M-mount lens system.

Most digicams are manufactured with a zoom lens that covers the most commonly used fields of view, with "super-zoom" models becoming more popular. Digicam lenses can be adapted to telephoto or wide-angle as the above-mentioned 'bridge-cameras'.

Digicams were once significantly slower in image capture (time measured from pressing the shutter release to the writing of the digital image to the storage medium) than DSLR cameras, but this situation is changing with the introduction of faster capture memory cards and faster in-camera processing chips. Currently, however, these cameras present a significant disadvantage for action, wildlife, sports and other photography requiring a high burst rate (frames per second).

The ability to switch lenses is shared by certain rangefinder cameras, both film and digital in operation. Two such digital rangefinders are the Epson R-D1 (possibly discontinued) and the Leica M8; both use sensors smaller than the full format of 35 mm rangefinder cameras.

• Category:SLR cameras
• Box camera
• Canon
• Digital photography
• Four Thirds System
• Fujifilm
• Kodak
• Nikon
• Optics
• Photographic film
• Rangefinder camera
• Scheimpflug principle
• Single-lens reflex camera
• Twin-lens reflex camera
• Zeiss Ikon

• [2] Camerapedia.org website.
• [3] Digital Photography Review
• [4] Outdoor Photographer
• [5] Popular Photography Magazine
• [6] Professional Photographer
• [7] Shutterbug Magazine website
• [8] Steve's Digicams
• [9] Vivid Light Photography
• [10] What Digital Camera