A pentaprism is a five-sided reflecting prism used to deviate a beam of

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

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 In human biology, handedness is an individual's preferential use of one hand, known as the dominant hand, due to and causing it to be stronger, faster or more Fine motor skill, dextrous. The other hand, comparatively often the weaker, less dext ...

) as an ordinary right-angle prism or mirror would. The reflections inside the prism are not caused by

total internal reflection In physics, total internal reflection (TIR) is the phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely refl ...

, 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, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...

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

A variant of this prism is the roof pentaprism which is commonly used in the viewfinder of

single-lens reflex camera In photography, a single-lens reflex camera (SLR) is a type of camera that uses a mirror and prism system to allow photographers to view through the lens and see exactly what will be captured. SLRs became the dominant design for professional a ...

s. 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 instrument used to capture and store images and videos, either digitally via an electronic image sensor, or chemically via a light-sensitive material such as photographic film. As a pivotal technology in the fields of photograp ...

'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 tempera ...

" 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 camera Medium format has traditionally referred to a film format in photography and the related cameras and equipment that use film. Nowadays, the term applies to film and digital cameras that record images on media larger than the used in 35&n ...

s, 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 Contax S with the world's first roof pentaprism on a

. Image:Nikonf.jpg,

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

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 Camera, Leica I Schraubgewinde (1930), Exakta (1936) and the Nikon F (1959). System cameras ar ...

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 (film), medium-format roll film cameras and photographic equipment based in Tokyo, Japan. Their single-lens reflex (SLR) system-cameras competed with ...

ETR

medium format Medium format has traditionally referred to a film format in photography and the related cameras and equipment that use film. Nowadays, the term applies to film and digital cameras that record images on media larger than the used in 35&n ...

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

In surveying

surveying Surveying or land surveying is the technique, profession, art, and science of determining the land, terrestrial Plane (mathematics), two-dimensional or Three-dimensional space#In Euclidean geometry, three-dimensional positions of Point (geom ...

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 direction as a reference line, or plumb-line. It is a precursor to the spirit level and used to esta ...

are used to stake out

right angle In geometry and trigonometry, a right angle is an angle of exactly 90 Degree (angle), degrees or radians corresponding to a quarter turn (geometry), turn. If a Line (mathematics)#Ray, ray is placed so that its endpoint is on a line and the ad ...

s, e.g. on a

construction Construction are processes involved in delivering buildings, infrastructure, industrial facilities, and associated activities through to the end of their life. It typically starts with planning, financing, and design that continues until the a ...

site.

References

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

In cameras

Compared to the pentamirror

In surveying

See also

References