Digital “SLR” stands for single lens reflex. An SLR camera lens that uses a mechanically controlled mirror system and prism to allow light to expose itself to the camera sensor only during the period of exposure right after the shutter is clicked. Digital SLR cameras are preferred by professional photographers because they feature interchangeable lenses and an accurate portrayal of what a shot will look like.

DSLR design principle
Phase-detection autofocus
DSLRs with live preview
DSLRs with HD video capture

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

Table of sensor sizes

The table lists dimensions of typical DSLR sensors.
Type
Four Thirds
Sigma Foveon
X3
Canon APS-C
Sony · Pentax · Sigma · Samsung
APS-C / Nikon DX
Canon APS-H
35mm Full-frame
/ Nikon FX
Leica S2
Pentax 645D
Phase One P 65+
Diagonal (mm)
21.6
24.9
26.7
28.2-28.4
33.5
43.2-43.3
54
55
67.4
Width (mm)
17.3
20.7
22.2
23.6-23.7
27.9
36
45
44
53.9
Height (mm)
13.0
13.8
14.8
15.6
18.6
23.9-24
30
33
40.4
Area (mm2)
225
286
329
368-370
519
860-864
1350
1452
2178
Crop factor
2.00
1.74
1.62
1.52-1.54
1.29
1.0
0.8
0.78
0.64
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 at a given aperture.
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/G10/G11/G12/S100 or the Nikon CoolpixP5000/P6000 use sensors that are approximately 5% and 4% of the area of a full frame sensor, respectively. The current exceptions are the Micro Four Thirds system by Olympus and Panasonic; the Sigma DP1, which uses a Foveon X3 sensor; the Leica X1; and the Canon PowerShot G1 X, which uses a 1.5″ (18.7 x 14mm) sensor that is slightly larger than the Four Thirds standard and is 30% of a fullframe sensor. Leica offers an “S-System” DSLR with a 30×45mm array containing 37 million pixels. This sensor is 56% larger than a full-frame sensor.

Depth-of-field control

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

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 2011, only a few current DSLRs have full-frame sensors, including the Sony α 900; the Canon EOS-1Ds Mark III and EOS 5D Mark II; andNikon D3X, D3S and 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.

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.
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.

Unusual features

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.
In August 2010 Sony released series of DSLRs allowing 3D photography. It was accomplished by sweeping the camera horizontally or vertically in Sweep Panorama 3D mode. The picture could be saved as ultra-wide panoramic image or as 16:9 3D photography to be viewed on BRAVIA 3D television set.

DSLR lenses

Interchangeable lenses for SLRs and DSLRs (also known as “Glass”) 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 EF lenses on a Canon body) although there are also many independent lens manufacturers, such as Sigma, Tamron, Tokina, and Vivitar, 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. 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 because of the smaller imaging circle. and, with some Canon EF-S lenses, interference with the reflex mirrors on full-frame bodies. Several manufacturers produce full-frame digital SLRcameras that allow lenses designed for the 35 mm film frame to operate at their intended angle of view.

Present-day models

Mainstream DSLRs (full-frame or smaller image sensor format) are produced by Canon, Nikon, Olympus, Pentax, Sigma, and Sony. Leica and Pentax produce single-lens reflex cameras with larger fixed sensors. Phase One, Hasselblad, and Mamiya Leaf produce expensive, high-end medium-format cameras with reflex mirrors and removable sensor backs. Contax, Fujifilm, Kodak, Panasonic, and Samsung previously produced DSLRs, but now either offer non-DSLR systems or have left the camera market entirely. Konica Minolta’s line of DSLRs was purchased by Sony.
§  Canon’s current 2012 EOS digital line includes the Canon EOS 1100D, 550D, 600D, 60D, 7D, 5D Mark II, 1Ds Mark III, and the 1D Mark IV. The 1Ds Mark III and 1D Mark IV will both be replaced in April 2012 by the 1D X, and the 5D Mark II will be replaced in late March 2012 by the 5D Mark III. As of March 2011, all current Canon DSLRs use CMOS sensors.
§  Leica produces the S2, which has a body similar to medium-sized DSLRs. However, in terms of sensor size and price, the camera is more like a medium-format camera.
§  Nikon has a broad line of DSLRs, most in direct competition with Canon’s offerings, including the D3100, D5100, D90, D7000, and D30

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Manohar(ECE 2nd  YEAR)