SLR stands for Single-lens Reflex, a type of camera design which involves the use of a reflex mirror to operate.
Originally all SLR cameras were film, but then the same design was used to develop digital cameras, which are classified as DSLRs – Digital Single-lens Reflex.
A reflex mirror (1) reflects light from the lens to a focusing screen (2). The image formed on the screen passes through a pentaprism (3) and is viewed though the optical eyepiece (4). When the shutter button is pressed, the mirror swings up to allow the light to pass through to the film or sensor (5) at the rear.
The first Canon EOS SLR camera was the EOS 650, launched in March 1987.
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