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,[1] 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 pentaprism.
A perspective drawing showing a roof pentaprism commonly used in a single lens reflex camera (SLR). The image is flipped laterally by the prism.

The reflections inside the prism are not caused by total internal reflection, since the beams are incident at an angle less than the critical angle (the minimum angle for total internal reflection). Instead, the two faces are coated to provide mirror surfaces. The two opposite transmitting faces are often coated with an antireflection coating to reduce spurious reflections. The fifth face of the prism is not used optically but truncates what would otherwise be an awkward angle joining the two mirrored faces.

In cameras
An image as seen through a roof pentaprism. This is looking in through the eyepiece plane.

A variant of this prism is the roof pentaprism which is commonly used in the viewfinder of single-lens reflex cameras.[1][2] 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.

Compared to the pentamirror

The same optical paths can be realized with three mirrors, in an arrangement called the pentamirror. While substantially lighter, the light enters and exits the mirrors' glass several times, each time losing brightness and instead scattering. The pentaprism is typically much heavier, but only has one entrance and one exit, providing a notably superior optical performance. Additionally, pentamirrors can conceivably go out of alignment while a pentaprism's facets are perfectly aligned until it is destroyed. [3]

In surveying
Main article: Optical square
Double pentaprism as used in surveying.

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.

See also
Hensoldt roof pentaprism binoculars

References
  1. Norman Goldberg (1992). Camera Technology. Academic Press. ISBN 0-12-287570-2.
  2. "Pentaprism: what is it and how does it work? - What Digital Camera". What Digital Camera. 14 April 2016. Retrieved 2018-05-12.
  3. David D. Busch. Digital Photography All-in-One Desk Reference for Dummies, 2006, page 106.
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