A camera based on the single-lens reflex (SLR) principle uses a mirror to show in a viewfinder the image that will be captured. The cross-section (side-view) of the optical components of an SLR shows how the light passes through the lens assembly is reflected into the pentaprism by the reflex mirror (which must be at an exact 45 degree angle) (1) and is projected on the matte focusing screen (4). Via a condensing lens (5) and internal reflections in the roof pentaprism (6) the image is projected through the eyepiece to the photographer's eye.
Focusing is either automatic, activated by pressing half-way on the shutter release or a dedicated AF button, as is mainly the case with an autofocusing film SLR; or manual, where the photographer manually focuses the lens by turning a lens ring on the lens barrel. When an image is photographed, the mirror swings upwards in the direction of the arrow, the focal-plane shutter (2) opens, and the image is projected and captured on the sensor (3), after which actions, the shutter closes, the mirror returns to a critical 45 degree angle, and the diaphragm reopens and the built in drive mechanism retensions the shutter for the next exposure. There is often a ring of soft material around the focusing screen, which helps to both cushion the impact of the mirror slapping up and help seal the mirror box from light entering through the eye piece.[1] Some high end cameras incorporate a shutter into the eyepiece to further eliminate light that may enter there during long exposures.
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