Digital single-lens reflex camera
Digital single-lens reflex camera
From Wikipedia, the free encyclopedia
A digital single-lens reflex camera (digital SLR or DSLR) is a digital camera that uses a mechanical mirror system and pentaprism to direct light from the lens to an optical viewfinder on the back of the camera.
The basic operation of a DSLR is as follows: for viewing purposes, the mirror reflects the light coming through the attached lens upwards at a 90 degree angle. It is then reflected twice by the pentaprism, rectifying it for the photographer's eye. During exposure, the mirror assembly swings upward, the aperture narrows (if stopped down, or set smaller than wide open), and a shutter opens, allowing the lens to project light onto the image sensor. A second shutter then covers the sensor, ending the exposure, and the mirror lowers while the shutter resets. The period that the mirror is flipped up is referred to as "viewfinder blackout". A fast-acting mirror and shutter is preferred so as to not delay an action photo.
All of this happens automatically over a period of milliseconds, with cameras designed to do this 3~10 times a second.
DSLRs are often preferred by professional still photographers because they allow an accurate preview of framing close to the moment of exposure, and because DSLRs allow the user to choose from a variety of interchangeable lenses. Most DSLRs also have a function that allows accurate preview of depth of field.
Many professionals also prefer DSLRs for their larger sensors compared to most compact digitals. DSLRs have sensors which are generally closer in size to the traditional film formats that many current professionals started out using. These large sensors allow for similar depths of field and picture angle to film formats.
The term DSLR generally refers to cameras that resemble 35 mm format cameras, although some medium format cameras are technically DSLRs.
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Brief difference between a DSLR and a digital point and shoot camera
The reflex design scheme is a major difference between a DSLR and an ordinary digital point and shoot camera, 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.
DSLR design principles
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 in a viewfinder the image that will be captured. 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.
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 must use contrast-detect autofocus instead, which is slower in some implementations.
[edit] DSLR optical viewfinder vs. digital point and shoot camera 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, many 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 macro photography and "micro-photography" (with a microscope).
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 while it is used 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 de-facto 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.
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 at the same time). 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. On most DSLRs, a disadvantage when using live preview is that the phase detection autofocus system does not work and the slower contrast system used in non DSLRs must be used.
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.
In late 2008[update], 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]
High Definition DSLRs (HDSLRs)
Introduced in 2008, HDSLRs are DSLRs which, in addition to taking still-photographs, offer a movie-mode capable of recording high definition motion-video. This feature parallels the evolution of compact digital cameras, many of which also offer HD-movie mode. The term HDSLR (High Definition Single Lens Reflex) first came into popular usage in April 2009 about the time of the debut of the ebook entitled HDSLR Nikon D90 by Peter iNova and Uwe Steinmueller.
The first HDSLR, the Nikon D90, captures video at 720p24 (1280x720 resolution at 24 fps) using an APS-sized imager. The more expensive Canon EOS 5D Mark II captures video at 1080p30 using a (35mm) full-size imager. Less than a year after the first HDSLR introduction, HD-movie mode has reached the entry-level models with the Canon EOS 500D (Rebel T1i) and Nikon D5000. The 500D supports both 720p30 and a limited 1080p mode which captures 20 fps. The D5000's movie-mode is comparable to the D90, with a maximum capture-mode of 720p24.
At PMA 2009 (Photo Marketing Association Trade Show) Panasonic gave the first public debut of a new combo-camera offering called the Lumix DMC-GH1, a new generation HDSLR offering greater video flexibility than previous competitors, while using a smaller Four Thirds-sized imager. The GH1 is the first HDSLR to offer full resolution HD (1920x1080) 1080p captured at the 24fps, the standard Academy frame-rate used by the motion picture industry. Also worth noting is that the Panasonic Lumix DMC-GH1 can autofocus while in shooting in movie-mode, a first for an HDSLR, and is currently the only HDSLR to support video-capture at 720p60 (or 720p50, for European models.) The GH1 became available for purchase in Japan in April 2009, and as of June 2009 is available worldwide in limited quantities.
On 05/20/2009, Pentax announced its K-7 HDSLR. It supports video-capture at 30 fps, in both 720p resolution, and an unusual high-resolution video mode of 1536×1024 which matches the 3:2 aspect ratio of the image sensor.
In July 2009, Olympus released the E-P1, which is the smallest and lightest HDSLR currently on the market. The E-P1 records video in 720p30 format.
DSLR lenses
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.
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 often 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 35 mm mount DSLRs, mostly in the wide angle range. These lenses tend not to be completely compatible with full frame sensors or 35 mm film.
Several manufacturers produce full-frame digital SLR cameras that allow lenses designed for the 35 mm film frame to operate at their intended angle of view. For more information about the dependence of angle of view on format size, see the article on crop factor.
DSLR design considerations
Pentaprism vs. penta-mirror
Most of the entry level DSLRs use a pentamirror instead of the traditional pentaprism. The pentamirror design is composed mostly of plastic and is lighter and cheaper to produce however the image in the viewfinder is usually darker.
Sensor size and image quality
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[update].
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, standard image format 24x36 mm); these sensors are used in quite expensive DSLRs such as the Canon EOS-1Ds Mark III, the Canon EOS 5D Mark II, the Nikon D700, the Nikon D3, the Nikon D3X, and the Sony Alpha 900. 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.
Leica offers an "S-System" DSLR with a 30x45mm sensor containing 37-million pixels.[9] This sensor is 56% larger than a full-frame 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.
Table of sensor sizes
The table lists dimensions of typical DSLR sensors.[10]
| Type | Four Thirds | Canon APS-C | Nikon DX | Canon APS-H | 35mm / Canon APS-F / Nikon FX | Leica S2 | Phase One P 65+ |
|---|---|---|---|---|---|---|---|
| Diagonal (mm) | 21.6 | 26.7 | 28.4 | 34.5 | 43.3 | 54 | 67.3 |
| Width (mm) | 17.3 | 22.2 | 23.6-.7 | 28.7 | 36 | 45 | 53.9 |
| Height (mm) | 13.0 | 14.8 | 15.5-.8 | 19.1 | 24 | 30 | 40.4 |
| Area (mm2) | 225 | 329 | 366-374 | 548 | 864 | 1350 | 1773 |
| Crop factor[11] | 2.00 | 1.62 | 1.52 | 1.26 | 1.0 | 0.8 | 0.64 |
Depth-of-field control
The lenses typically used on DSLRs have a wider range of apertures available to them, ranging from as large as f/1.0 to about f/32. Lenses for digicams rarely have true available aperture sizes much larger than f/2.8 or much smaller than f/5.6.
The f/5.6 limitation is because lens designs of typical small sensor digicams already produce diffraction blur bigger than a few pixels at f/5.6.[12] Because of digicams' smaller sensors there are a limited number of apertures available that will produce an acceptably sharp image. Many digicams only have a two-stop range of apertures because at settings outside of these the image will become too soft because of limits of lens design at large apertures, or diffraction at smaller apertures. To help extend the exposure range, some digicams will also incorporate an ND filter pack into the aperture mechanism.[13]
The apertures that digicams have available give much more depth of field than equivalent angles of view on a DSLR. For example a 6 mm lens on a 2/3" sensor digicam has a field of view similar to a 24 mm lens on a 35 mm camera. At an aperture of f/2.8 the digicam (assuming a crop factor of 4) has a similar depth of field to that 35 mm camera set to f/11 – that's a four-stop difference. Put another way, with both cameras at f/2.8 and focused on a subject 1 meter from the camera, and both cameras zoomed to produce the same angle of view (35 mm camera will need to use larger focal length to produce same angle of view from same distance), the digicam might have a depth of field of 2 meters and the larger camera would have a depth of field of 0.3 meters.[14][15]
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 35 mm film format (36 mm × 24 mm frame) gives a narrower angle of view for a lens of a given focal length than a camera equipped with a full-frame (35 mm) sensor. As of 2008, only a few current DSLRs have full-frame sensors, including the Sony α 900, Canon EOS-1Ds Mark III, 5D Mark II, Nikon D3 and Nikon D700. 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 (35 mm) sensors, and capable of even greater resolution, and are correspondingly 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 50 mm will give a field of view equal to that of a 75 mm to 85 mm 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.
DSLRs with "crop" sensor size have slightly more depth-of-field than cameras with 35 mm sized sensors for a given angle of view. The amount of added depth of field for a given focal length can be roughly calculated by multiplying the depth of field by the crop factor. Shallower depth of field is often preferred by professionals for portrait work and to isolate a subject from its background.
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.
Dust reduction systems
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.[16]
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.
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 21–60 megapixels as of July 2009). 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 moiré.[17][18]
As of 2007 integrated medium formats like the Phase One 645 system[19], Hasselblad H System[20] and Leaf AFi[21] have started to appear.
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.[22]
History
On August 25, 1981 Sony unveiled a prototype of the first still video camera, the Sony Mavica. This camera was an analog electronic camera that featured interchangeable lenses and a SLR viewfinder.
At Photokina in 1986, Nikon revealed a prototype analog electronic still SLR camera, the Nikon SVC, a precursor to the digital SLR.[23] The prototype body shared many features with the N8008.[23]
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.[24]
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 35 mm 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 EOS-1D, 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 of focus, higher frame rates, the elimination of digital 'noise' produced by the imaging sensor, and price reductions to lure new customers.
Market share
As of 2008[update], 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.[25] In the Japanese domestic market, Nikon captured 43.3% to Canon's 39.9%, with Pentax a distant third at 6.3%.[26]
The duopoly of Canon and Nikon is sometimes referred to as "Canikon" or "Nikanon" in online forums in skeptical challenge to the presumptive acceptance of these manufacturer's cameras as always "the best". Canon and Nikon have used their professional market presence especially persuasively in the sale of entry level offerings to the uninitiated general public who presume that everything from Canon or Nikon is superlative.[citation needed] Online contributors often challenge the "Canikon/Nikanon" supposed superiority when they believe there are superior innovations from the smaller DSLR manufacturers.
The DSLR market is dominated by Japanese companies, including all of the top five manufacturers (Canon, Nikon, Olympus, Pentax, and Sony), as well as Fujifilm, Mamiya, and Sigma. Leica is German, Hasselblad is Swedish, and Samsung is Korean, while the American company Kodak formerly produced DSLRs as well.
Present-day models
Mainstream DSLRs (full-frame or smaller image sensor format) are currently produced by Canon, Fujifilm, Leica, Nikon, Olympus, Panasonic, Pentax, Samsung, Sigma, and Sony. Hasselblad and Mamiya also produce expensive, high-end medium-format DSLRs.
- Canon's current EOS digital line includes the Next post: