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Almost all SLR used a front surfaced mirror in the optical path to direct the light from the lens via a viewing screen and pentaprism to the eyepiece.

At the time of exposure the mirror flipped up out of the light path before the shutter opened.

Some early cameras experimented other methods of providing through the lens viewing including the use of a semi transparent pellicle as in the Canon Pellix and others with a small periscope such as in the Corfield Periflex series.