Digital single-lens reflex cameras (DSLR) are cameras that use a mechanical mirror system and pentaprism to direct light from the lens to an optical viewfinder on the back of the camera. The basic operation of a DSLR is as follows: for viewing purposes, the mirror reflects the light coming through the attached lens upwards at a 90 degree angle. It is then reflected three times by the roof pentaprism, rectifying it for the photographer’s eye.
During exposure, the mirror assembly swings upward, the aperture narrows (if stopped down, or set smaller than wide open), and a shutter opens, allowing the lens to project light onto the image sensor. A second shutter then covers the sensor, ending the exposure, and the mirror lowers while the shutter resets. All of this happens automatically over a period of milliseconds, with cameras designed to do this 3–10 times per second.
The Daily Omnivore 
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