Pentaprism

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.A variant of this prism is the roof pentaprism which is commonly used in the viewfinder of single-lens reflex cameras. The camera lens renders an image that is both vertically and laterally reversed, and the reflex mirror re-inverts it leaving an image laterally reversed. In this case, the image needs to be reflected left-to-right as the prism transmits the image formed on the camera's focusing screen. This lateral inversion is done by replacing one of the reflective faces of a normal pentaprism with a 'roof' section, with two additional surfaces angled towards each other and meeting at 90°, which laterally reverses the image back to normal. Reflex cameras with waist-level finders (viewed from above), including many medium format cameras, display a laterally reversed image directly from the focusing screen which is viewed from above.Zeiss Ikon Contax S with the world's first roof pentaprism on a single-lens reflex camera.Nikon F with an interchangeable roof pentaprism — the first system camera with a roof pentaprism.The standard DP-30 prism of a Nikon F5, removed.Prism housing for Bronica ETR medium format cameras.A cutaway view of a Praktica L2 showing the pentaprism.In surveying a double pentaprism (two pentaprisms stacked on top of each other) and a plumb-bob are used to stake out right angles, e.g. on a construction site.