 Almost all SLR cameras use 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 is flipped up out of the light path before the shutter opens.Some early cameras experimented with 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.