Pentaprism Pins

A pentaprism is a five-sided reflecting prism used to deviate a beam of light by a constant 90°, even if the entry beam is not at 90° to the prism. The beam reflects inside the prism twice, allowing the transmission of an image through a right angle without inverting it (that is, without changing the image's handedness) as an ordinary right-angle prism or mirror would.

Before pentaprisms, photographers had to look down when taking photos, which was limiting and only allowed for hip-level shots. Pentaprisms are considered a defining feature of SLR cameras and are thought to have been essential to their success.