Digital single-lens reflex camera

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

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, and a shutter opens, allowing the lens to project light onto the image sensor.

Contents 1 Brief difference between a DSLR and a Digicam 2 DSLR design principles 2.1 Fast phase-detection autofocus 2.2 DSLR optical viewfinder vs. digicam LCD 2.3 DSLRs with live preview 3 DSLR lenses 3.1 Lens mounts and lens manufacturers 4 DSLR design considerations 4.1 Pentaprism vs. penta-mirror 4.2 Sensor size and image quality 4.3 Depth-of-field control 4.4 Angle of view 4.5 Mode dial 4.6 Dust reduction systems 4.7 Medium format digital 4.8 Unusual features – infrared and ultraviolet photography 5 History 6 Present day 7 DSLRs compared to other digital cameras 7.1 Fixed-lens cameras 7.1.1 SLR-like cameras – "bridge cameras" 7.1.2 Digicams (compact 'point-and-shoot' digital cameras) 7.2 Non-SLR interchangeable lens digital cameras 8 See also 9 References 10 External links

[edit] Brief difference between a DSLR and a Digicam

The reflex design scheme is a major difference between a DSLR and 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 liquid crystal display (LCD) before the photograph is taken. However, many newer DSLR models feature live preview, allowing the LCD 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.

[edit] DSLR design principles

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 through the eyepiece (8) to the photographer's eye. Focusing is either 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 swings 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. There is often a ring of soft material around the focusing screen, which helps to both cushion the impact of the mirror slapping up and help seal the mirror box from light entering through the eye piece.^[1] Some high end cameras incorporate a shutter into the eyepiece to further eliminate light that may enter there during long exposures.

[edit] Fast phase-detection autofocus

The diagram shown here is an over-simplification in that it omits the sensors used to activate the drive for the autofocus system. Those sensors reside at the bottom of the mirror box. In such a system, the main mirror is slightly translucent in the center, which allows light to pass through it to a secondary mirror which reflects light to the sensors below.

DSLRs typically use a phase detection autofocus system. This method of focus is very fast, and results in less focus "searching", but requires the incorporation 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.

[edit] DSLR optical viewfinder vs. digicam LCD

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 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 LCDs 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 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 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. Electronic viewfinders may also provide a brighter display in low light situations, as the picture can be electronically amplified; conversely, LCDs can be difficult to see in very bright sunlight.

[edit] DSLRs with live preview

A fairly recent development in DSLRs is the increased availability of live preview options, which make it possible to use either the optical viewfinder or the LCD 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 it is likely that other new DSLRs will eventually have this feature.

As of early 2008, some DSLRs from Canon, Nikon, Olympus, Panasonic, Leica, Pentax, Samsung and Sony all provide continuous live preview as an option. Additionally, the Fujifilm FinePix S5 Pro^[2] offers 30 seconds of live preview.

Some live preview systems make use of the primary sensor to provide the image on the LCD (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.^[3]

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.^[4]

[edit] DSLR lenses

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.

[edit] Lens mounts and lens manufacturers

Interchangeable lenses for SLRs and DSLRs are built to operate correctly with a specific lens mount that is generally unique to each brand. A photographer will often use lenses made by the same manufacturer as the camera body (for example, Canon lenses on a Canon body) although there are also many independent lens manufacturers, such as Sigma,^[5] Tamron,^[6] Tokina,^[7] and Vivitar,^[8] 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.

Most DSLR manufacturers have introduced lines of lenses with image circles and focal lengths optimized for the smaller sensors generally offered for existing 35mm mount DSLRs, mostly in the wide angle range. These lenses tend not to be completely compatible with full frame sensors or 35mm film.

[edit] DSLR design considerations

[edit] Pentaprism vs. penta-mirror

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

[edit] Sensor size and image quality

Main article: Image sensor format

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.

With the exception of 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.

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. The one current exception is the Sigma DP1, which uses a Foveon X3 sensor.

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.

[edit] Depth-of-field control

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 DSLRs 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.

[edit] Angle of view

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 III, 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. Typical APS-C sensors have crop factors 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 35 mm camera. The smaller sensors of Four Thirds system cameras have a crop factor of 2.0.

While the crop factor of APS-C cameras effectively narrows the angle of view of long-focus (telephoto) lenses, making it easier to take close-up images of distant objects, wide-angle lenses suffer a reduction in their angle of view by the same factor. Also, it is more difficult to achieve shallow depth-of-field (often desirable in portraiture) with an APS-C camera, since smaller focal lengths and smaller image sensor sizes both increase the depth of field, for a given f-number and camera-to-subject distance.

[edit] Mode dial

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.

[edit] Dust reduction systems

Main article: Dust reduction system

The fact that it is possible to change lenses on a DSLR results in the possibility of dust entering the camera body and adhering to the image sensor. This can reduce image quality, and make it necessary to clean the sensor. Various techniques exist including using a cotton swab with various fluids or blowing with compressed air. Some people prefer to clean the sensor themselves and some send the camera in for service.^[9]

A method to prevent dust entering the chamber, by using a "dust cover" filter right behind the lens mount, was pioneered by Sigma in their first DSLR, the Sigma SD9, in 2002.

Olympus pioneered a built-in sensor cleaning facility in their first DSLR that had a sensor exposed to air, the Olympus E-1, in 2003. Other DSLR manufacturers followed suit, and dust reduction systems are becoming common in DSLRs. There is some controversy as to how effective these systems are; see dust reduction system for more information.

[edit] Medium format digital

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. Another potential disadvantage of medium format digital backs is that there are none currently available (as of early 2008) that incorporate a low-pass (aka optical anti-aliasing filter) except for the Mamiya ZD, which has a removable one. This is done to allow the maximum resolution to be extracted from a given image, but at the cost of moire.^{[10] [11]}

As of 2007 integrated medium formats like the Hasselblad H System^[12] and Leaf AFi^[13] have started to appear.

[edit] Unusual features – infrared and ultraviolet photography

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 spectra of light.^[14]

[edit] History

At Photokina in 1986, Nikon revealed a prototype analog electronic still SLR camera, the Nikon SVC, a precursor to the digital SLR.^[15] The prototype body shared many features with the N8008.^[15]

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.^[16]

Over the next decade, DSLRs have been released by various companies, including Canon, Nikon, Kodak, Pentax, Olympus, Panasonic, Samsung, Minolta (later Konica Minolta, and whose camera assets were then acquired by Sony), Fujifilm, and Sigma, with higher resolutions and lower prices.

In 1999, Nikon announced the Nikon D1, the first DSLR to truly compete with, 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 November 2001, Canon released its 4.1 megapixel 1D Mark I, the brand's first professional digital body.

In 2003, Canon introduced the 6.3 megapixel EOS 300D SLR camera (known in the United States as the Digital Rebel and in Japan as the Kiss Digital) with an MSRP of US$999, directed at the consumer market. Its popularity encouraged other manufacturers to produce affordable digital SLR cameras, lowering entry costs and allowing more amateur photographers to purchase DSLRs.

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.

[edit] Present day

For timelines of DSLR models, see Canon DSLR cameras, Konica Minolta/Sony DSLR cameras, Nikon DSLR cameras, Olympus DSLR cameras, and Pentax DSLR cameras.

As of 2008, DSLR sales are dominated by Canon's and Nikon's offerings. For 2007, Canon edged out Nikon with 41% of worldwide sales to the latter's 40%, followed by Sony and Olympus each with approximately 6% market share.^[17] In the Japanese domestic market, Nikon captured 43.3% to Canon's 39.9%, with Pentax a distant third at 6.3%.^[18] The duopoly of Canon and Nikon is sometimes disparagingly referred as "Canikon" or "Nikanon" in online forums to suggest a lack of product differentiation and innovation by these manufacturers.

DSLRs are currently produced by Canon, Fujifilm, Leica, Nikon, Olympus, Panasonic, Pentax, Samsung, Sigma, and Sony. Hasselblad and Mamiya also produce expensive, professional medium-format DSLRs.

• Canon's current EOS Digital line includes the 450D, 40D, 5D and the 1D Mark III. Canon's latest cameras, the 40D and the full-frame 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, D60, D80, D300, and D3. The latter, announced in August 2007,^[19] is the company's first full-frame digital SLR.

• Fujifilm currently sells the Fujifilm FinePix S5 Pro DSLR, compatible with the Nikon F-mount lens system. It is is based on the Nikon D200 camera body, but utilizes Fuji's sensor technology (Fujifilm SuperCCD SR Pro) and menu system. Fuji previously offered 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-420, E-330, 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. The three companies share technology to a significant degree.

• Pentax (in collaboration with Samsung) currently offers the Pentax K200D and K20D, while Samsung offers the Samsung GX-20, a clone of the K20D, and the GX-10, a clone of the now-discontinued Pentax K10D. Innovative features include in-body image stabilization, dust reduction system, use of standard AA batteries in the K200D, weather-proof sealing (first introduced on the K10D, and otherwise found only in more expensive semi-pro models like the Nikon D200), and adoption of Adobe's DNG standard raw image format. Also, they offer extensive backwards compatibility, accepting all Pentax 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 DSLRs under the Sony α brand. They currently offer the 200, α 300, α 350, and α 700. The α series offers in-body image stabilization and retains the Minolta AF lens mount.

• Sigma produces DSLRs using the Foveon X3 sensor, rather than the conventional Bayer sensor. This is claimed to give higher colour resolution although headline pixel counts are lower than conventional Bayer-sensor cameras. Their current model is the Sigma SD14. Sigma is the only DSLR manufacturer which sells lenses for other brands' lens mounts.

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

[edit] DSLRs compared to other digital cameras

[edit] Fixed-lens 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^[20] (and often higher-quality) lenses.
• Image sensors of much larger size and often higher quality, offering lower noise,^[21] which is useful in low light, and greater dynamic range.^[22]
• 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.^[23]
• The larger focal length for the same field of view allows creative use of depth of field effects.^[24]
• Ability to attach additional accessories ^[25] 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.^[26]
• Generally greater cost.^[27]
• Lack of a video mode.^[28] No DSLRs produced so far can record full-motion video, while this has become a standard feature of compact digital cameras.
• Most DSLR models lack live preview on their LCDs, a nearly universal feature among compact digital cameras (some newer DSLRs do offer this).
• Noisier operation, due to the SLR mirror mechanism.^[29]
• 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.^[30]

[edit] SLR-like cameras – "bridge cameras"

Main article: Bridge digital camera

The "SLR-like" or "advanced" digicams 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. 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 (that is, it reacts with a delay 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.

Bridge digital cameras with their fixed lenses aren't usually subject to dust from outside the camera settling on the sensor. However having fixed lenses they are limited to the focal lengths they are built with, except for what is available from attachments. Current designs are limited by increasingly high pixel pitches, which limit their dynamic range and also call for increasingly higher quality lens designs. Exceptions to this trend are the Sigma DP1 with it's 20.7x13.8mm sensor and the Sony DSC-R1^[31] with a 21.5 x 14.4 mm sensor.

[edit] Digicams (compact 'point-and-shoot' digital cameras)

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 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).

[edit] Non-SLR interchangeable lens digital cameras

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.

[edit] See also

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

[edit] References

[edit] External links

• [1] Camerapedia.org website.
• [2] Camera Labs
• [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