A digital single-lens reflex camera (digital SLR or DSLR) is a digital camera that uses an automatic mirror system and pentaprism or pentamirror to direct light from the lens through the viewfinder eyepiece.
The basic operation of a DSLR is as follows: for viewing purposes, the mirror reflects the light coming through the attached lens upwards at an approximately 90 degree angle. It is then reflected by the pentaprism to the photographer's eye. During exposure (when the photograph is taken), the mirror swings upward, and a shutter opens, allowing the lens to project light onto the image sensor.
Difference between DSLR camera and digital camera (Digicam)
The reflex design scheme is a major difference between a DSLR and an ordinary digital camera (digicam), 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.
In most other respects, a DSLR is similar in principle and operation to a standard (non-SLR) digital camera.