Difference in SLR and DSLR

Advantages

Many of the advantages of SLR cameras derive from viewing and focusing the image through the attached lens. Most other types of cameras do not have this function; subjects are seen through a viewfinder that is near the lens, making the photographer's view different from that of the lens. SLR cameras provide photographers with precision; they provide a viewing image that will be exposed onto the negative exactly as it is seen through the lens. There is no parallax error, and exact focus can be confirmed by eye — especially in macro photography and when photographing using long focus lenses. The depth of field may be seen by stopping down to the attached lens aperture, which is possible on most SLR cameras except for the least expensive models. Because of the SLR's versatility, most manufacturers have a vast range of lenses and accessories available for them.
Compared to most fixed-lens compact cameras, the most commonly used and inexpensive SLR lenses offer a wider aperture range and larger maximum aperture (typically f/1.4 to f/1.8 for a 50 mm lens). This allows photographs to be taken in lower light conditions without flash, and allows a narrower depth of field, which is useful for blurring the background behind the subject, making the subject more prominent. 'Fast' lenses are commonly used in theater photography, portrait photography, surveillance photography, and all other photography requiring a large maximum aperture.
The variety of lenses also allows for the camera to be used and adapted in many different situations. This provides the photographer with considerably more control (i.e., how the image is viewed and framed) than would be the case with a view camera. In addition, some SLR lenses are manufactured with extremely long focal lengths, allowing a photographer to be a considerable distance away from the subject and yet still expose a sharp, focused image. This is particularly useful if the subject includes dangerous animals (e.g., wildlife); the subject prefers anonymity to being photographed; or else, the photographer's presence is unwanted (e.g., celebrity photography or surveillance photography). Practically all SLR and DSLR camera bodies can also be attached to telescopes and microscopes via an adapter tube to further enhance their imaging capabilities

Disadvantages

In most cases, single-lens reflex cameras cannot be made as small or as light as other camera designs — such as rangefinder cameras, autofocus compact cameras and digital cameras with electronic viewfinders (EVF) — owing to the mirror box and pentaprism/pentamirror. The mirror box also prevents lenses with deeply-recessed rear elements from being mounted close to the film or sensor unless the camera has a mirror lockup feature; this means that simple designs for wide angle lenses cannot be used. Instead, larger and more complex retrofocus designs are required.

During an exposure the viewfinder is blocked

The SLR mirror 'blacks-out' the viewfinder image during the exposure. In addition, the movement of the reflex mirror takes time, limiting the maximum shooting speed. The mirror system can also cause noise and vibration. Partially-reflective (pellicle) fixed mirrors avoid these problems and have been used in a very few designs including the Canon Pellix and the Canon EOS-1N RS, but these designs introduce their own problems. These pellicle mirrors reduce the amount of light travelling to the film plane or sensor and also can distort the light passing through them, resulting in a less-sharp image. To avoid the noise and vibration, many professional cameras offer a mirror lock-up feature, however, this feature totally disables the SLR's automatic focusing ability. Electronic viewfinders have the potential to give the 'viewing-experience' of a DSLR (through-the-lens viewing) without many of the disadvantages.

 Reliability of SLRs

SLRs vary widely in their construction and typically have bodies made of plastic or magnesium. Most manufacturers don't cite durability specifications, but some report shutter life expectancies for professional models. For instance, the Canon EOS 1Ds MkII is rated for 200,000 shutter cycles and the newer Nikon D3 is rated for 300,000 with its exotic carbon fiber/kevlar shutter. Because many SLRs have interchangeable lenses, there is a tendency for dust, sand and dirt to get into the main body of the camera through the mirror box when the lens is removed, thus dirtying or even jamming the mirror movement mechanism or the shutter curtain mechanism itself. In addition, these particles can also jam or otherwise hinder the focusing feature of a lens if they enter into the focusing helicoid. The problem of sensor cleaning has been somewhat reduced in DSLRs as some cameras have a built-in sensor cleaning unit.