DSLR stands for Digital Single Lens Reflex. The name actually describes the way the camera works. A single lens is used to feed both the camera sensor, the CCD that captures the image, and the viewfinder.

With most cheaper pocket cameras there are two lenses. One is connected to the camera sensor and one is connected to the viewfinder through which the photographer can compose the photo.

With DSLR cameras like the Canon Digital SLR most of the time when the photographer needs to view the scene the mirror is positioned to reflect the light from the camera single lens upwards in ninety degrees to the camera viewfinder. The photographer thus sees the exact picture that the camera sensor will see when the picture is taken thus using the viewfinder is more accurate and the photo composition is better. When the photographer presses the camera shutter button to take the photo the mirror moves away from the optical lens path and the light is thus not diverted anymore. Instead the light coming in through the lenses falls right on the camera sensor. The mirror moves for a short time just as much as is needed for the photo to be captured. The mechanical movement of the mirror generates a familiar photo shoot sound that some pocket cameras mimic using a speaker and a recorded sound.

One side effect of the single lens reflex and the mirror design is that when the mirror diverts light to the viewfinder the camera sensor is blind. The result is that the camera LCD screen can only be used to review photos already taken earlier and stored on the camera memory. Live scene feed is not available on the LCD screen since it is not available to the CCD sensor since the light instead goes to the viewfinder. With pocket cameras light always falls on the camera CCD sensor and thus the LCD can be used as a viewfinder allowing live feed of the scene seen by the camera.

The problem of not being able to use the LCD screen for photo composition was solved in newer digital SLR cameras. These Digital SLR cameras support live feed to the LCD by using one of two techniques. Some DLSR cameras replace the simple mirror with an optical element the breaks the light into two paths. Most of the light energy goes to the optical viewfinder while a small part of it goes to the camera sensor to support LCD live view. Another design is keeping the same mirror but adding a secondary small CCD sensor inside the viewfinder that captures the live feed. The secondary CCD can be small and of lower quality since the LCD live view is used for photo composition and its quality is not that important.