Digital SLR

A digital single-lens reflex camera (digital SLR or DSLR) is a digital camera that operates on the same optical and mechanical principles as a modern electronic autofocus 35mm film single-lens reflex camera. more...

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The key difference is that the film is replaced with a CCD or CMOS image sensor plus accompanying electronics, thus creating images digitally in-camera, without the need to first chemically develop a latent image on film.

SLR design

The single-lens reflex (SLR) camera 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 by the mirror (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 appears in the eyepiece (8). When an image is taken, the mirror moves in the direction of the arrow, the focal-plane shutter (3) opens, and the image is projected in the sensor (4) in exactly the same manner as on the focusing screen.

Parallax-free optical viewfinder

The principal advantage of DSLR cameras over other digital cameras is the defining characteristic of an SLR: the image in the optical viewfinder is parallax-free, because its light is routed directly from the main lens itself, rather than from an off-axis viewfinder.

However this advantage of seeing the image exactly as it will be captured has been duplicated in many digital compact cameras in their LCD displays. The SLR still retains an advantage because the LCD images are electronically mediated and thus possess time lag, and also have relatively low resolution and brightness making it difficult to see details, especially in outdoor use. In contrast, the SLR's optical image is in real-time and is brighter and more detailed.

Fast phase-detection autofocus

Like most film SLRs, DSLRs typically use a phase detection autofocus system. This method of focus is very fast, and results in less focus "searching", but requires the insertion of a special sensor into the optical path, so it is usually only used in SLR designs. Compact cameras 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.

Interchangeable 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, though not inherently unique to DSLRs. The diameters of the lenses for DSLRs are generally larger than for compact point-and-shoots, resulting in more light captured from the subject, allowing the option of using faster shutter speeds, especially in the case of low-light or moving subjects.

Read more at Wikipedia.org