DSLR
A digital single lens reflex or DSLR camera is a single-lens reflex camera (SLR) which records images using an electronic sensor (Usually a CCD or CMOS chip) instead of film. Like SLRs, DSLRs are often used by professional photographers and advanced amateurs.
DSLR accepts interchangeable lenses and accessories such as flash units, and accessories for SLRs are usually compatible with DSLRs of the same brand. When aiming a DSLR, the photographer is looking through the lens (unlike a rangefinder camera or a camera with a separate viewfinder) and the image is projected optically (unlike a camera with an electronic viewfinder).
The imaging sensor in a DSLR is typically much larger than the one in a standard digital camera. Larger sensor allows better image quality, shallower depth of field and wider field of view. However, the sensor is usually smaller than a frame of 135 film. The average size of a typical DSLR sensor being about 22 mm x 15 mm, or the size of an APS film frame. If this is not the case and the sensor is the same size as the equivalent frame of film (usually 35 mm wide), the camera is said to have a full-frame sensor. Currently (2005) only some expensive professional DSLRs have full-frame sensors (typically 36 mm x 24 mm). One example of a full-frame DSLR is the Canon EOS-1Ds Mark II.
Depending on the position of the reflex mirror, the light from the scene can only reach either the eyepiece or the sensor. Also, a mechanical shutter is used to control the exposure. Therefore even though a DSLR has a color display at the back of the camera body, it doesn't provide "live preview" (allowing focusing and framing using the display). The only exception to this rule so far is Canon EOS 20Da, a Japanese variant of Canon EOS 20D designed for astrophotography.
Camera manufacturers expect that professionals and advanced amateurs continue to prefer DSLRs over other forms of digitals cameras. However, if electronic viewfinders with near-zero lag and very low energy consumption are introduced, non-SLR cameras may become a viable choice for a large number of people who now need SLRs.
Many medium format SLRs can accept a digital camera back to turn the camera into a kind of DSLR with very high image quality. However, the combination is very expensive and bulky, and thus they are rarely used outside fashion photography and portraiture.
Digital SLR versus Digital compacts
What is the edge of Digital SLRs over non-SLR digital cameras?
Non-SLR digitals are of 2 types, digital compacts and SLR-like "prosumer"compacts (e.g., Minolta DimageZ2, Nikon 5700). Prosumer digitals have lenses with large zoom capacity (10x-12x) which even vies with some DSLRs.
In the conventional film photography, the SLR method's biggest advantage over Rangefinder method is TTL (Through The Lens) viewing of the image to be photographed. In a Rangefinder camera, the frame is seen via a prism, not through the focusing lens. However, most Prosumer digitals (like Nikon 5700) also offer TTL viewing through the focusing lens, projected onto its viewfinder as well as the LCD screen,through an EVF(electronic viewfinder). This differs from the DSLR in that this is a digitally created image whereas a DSLR gives the real optical TTL image. An EVF image is slower to readjust to a change and of lower resolution than a true optical TTL. Advantages of EVF are no parallax error and lesser bulk.
Entry level compact digital cameras offer only the prism viewfinder viewing.
A DSLR camera can show you the true optical image its viewfinder. Queerly, the LCD screen of a DSLR shows the snap only after you have shot it, but not before clicking. DSLRs allows for easy lens interchanges, which is not available in any non-SLR digitals. (High end rangefinder cameras of conventional film photography allows lens interchange.) DSLR is also faster (time from the clicking to sensor and then to store the snap onto the memory card) than digital compacts which is helpful in action photography.SLR also gives more control on aperture and shutter speed, which matters for artistic photography.
The bottom line is that DSLRs combine all the plus points of both coventional and digital photography. The main drawbacks are the bulk of the equipment and cost.
DSLR also suffers "dust over the sensor" problem.Because of the interchangeabilty of lenses, environmental dust accumulates on the thin glass cover in front of the sensor.It has to be cleaned periodically to avoid surprising blotches in the enlarged print outs.If not properly performed,permanent scratches will be etched on the glass cover and affects the clarity.