Pentaprism

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A Pentaprism is a five-sided optical glass element that reflects light through 90°. The variant used in most SLR's (and pentaprism finders for system TLR's) is the roof pentaprism, which has two sloping sides on one of the reflecting surfaces, usually the top, to allow light from the mirror to be reflected into an eye-level (reflex) viewfinder with the picture appearing the correct way around to the photographer. This avoids the problem suffered by other reflex viewfinder systems, which can have the image laterally (left-right) reversed - or even upside-down, making framing difficult, and following a moving subject confusing. The roof pentaprism is the reason for most SLR cameras having a triangular top to their viewfinders.

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