Pentaprism

A Pentaprism is a five-sided optical glass element that reflects light through 90°. A variant, the roof pentapism has an two sloping sides at the top to allow light from the mirror of an SLR 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, making following a moving subject difficult or confusing. The roof pentaprism is the reason for most SLR cameras having a triangular top to their viewfinders.

Links

• Viewfinder Types on Ted's Photographics by Ted Ellis
• Wikipedia Pentaprism article