A pentaprism is a five-sided reflecting

prism Prism usually refers to: * Prism (optics), a transparent optical component with flat surfaces that refract light * Prism (geometry), a kind of polyhedron Prism may also refer to: Science and mathematics * Prism (geology), a type of sedimentary ...

used to deviate a beam of

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

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. 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 A mirror or looking glass is an object that reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the im ...

surfaces. The two opposite transmitting faces are often coated with an

antireflection coating An antireflective, antiglare or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses, other optical elements, and photovoltaic cells to reduce reflection. In typical imaging systems, this improves the effici ...

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

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 A camera is an optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), with a ...

's focusing screen. This lateral inversion is done by replacing one of the reflective faces of a normal pentaprism with a "

roof A roof ( : roofs or rooves) is the top covering of a building, including all materials and constructions necessary to support it on the walls of the building or on uprights, providing protection against rain, snow, sunlight, extremes of te ...

" section, with two additional surfaces angled towards each other and meeting at 90°, which laterally reverses the image back to normal.

Reflex camera {{Unreferenced, date=July 2021 A reflex camera is a camera that permits the photographer to view the image that will be seen through the lens, and therefore to see exactly what will be captured, contrary to viewfinder cameras where the image coul ...

s 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. David D. Busch. ''Digital Photography All-in-One Desk Reference for Dummies'', 2006
page 106
Image:Contax-s.jpg,

Zeiss Ikon Carl Zeiss AG (), branded as ZEISS, is a German manufacturer of optical systems and optoelectronics, founded in Jena, Germany in 1846 by optician Carl Zeiss. Together with Ernst Abbe (joined 1866) and Otto Schott (joined 1884) he laid the ...

Contax S with the world's first roof pentaprism on a single-lens reflex camera. Image:Nikonf.jpg,

Nikon F The Nikon F camera, introduced in April 1959, was Nikon's first SLR camera. It was one of the most advanced cameras of its day. Although many of the concepts had already been introduced elsewhere, it was revolutionary in that it was the firs ...

with an interchangeable roof pentaprism — the first

system camera A system camera or camera body is a camera with interchangeable components that constitutes the core of a system. Early representatives include Leica I Schraubgewinde (1930), Exakta (1936) and the Nikon F (1959). System cameras are often sin ...

with a roof pentaprism. Image:Nikon F5 Prism 2485.jpg, The standard DP-30 prism of a Nikon F5, removed. Image:Bronica-ETR-Finder.jpg, Prism housing for

Bronica Bronica also Zenza Bronica (in Japanese: ) was a Japanese manufacturer of classic medium-format roll film cameras and photographic equipment based in Tokyo, Japan. Their single-lens reflex (SLR) system-cameras competed with Pentax, Hasselblad, Ma ...

ETR medium format cameras. File:Pentaprisma-kol.jpg, A cutaway view of a Praktica L2 showing the pentaprism.

In surveying

In surveying a double pentaprism (two pentaprisms stacked on top of each other) and a

plumb-bob A plumb bob, plumb bob level, or plummet, is a weight, usually with a pointed tip on the bottom, suspended from a string and used as a vertical reference line, or plumb-line. It is a precursor to the spirit level and used to establish a vertic ...

are used to stake out right angles, e.g. on a

construction Construction is a general term meaning the art and science to form Physical object, objects, systems, or organizations,"Construction" def. 1.a. 1.b. and 1.c. ''Oxford English Dictionary'' Second Edition on CD-ROM (v. 4.0) Oxford University Pr ...

site.

References

{{Reflist Prisms (optics) Prismatoid polyhedra Photography equipment Surveying instruments

In cameras

Compared to the pentamirror

In surveying

See also

References