A pentamirror is an optical device used in the

viewfinder In photography, a viewfinder is what the photographer looks through to compose, and, in many cases, to focus the picture. Most viewfinders are separate, and suffer parallax, while the single-lens reflex camera lets the viewfinder use the main ...

systems of various

single-lens reflex A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system (hence "reflex" from the mirror's reflection) that permits the photographer to view through the lens and see exactly what will be captured. With twin le ...

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 instead of the

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 r ...

. It is used to reverse again the laterally reversed image coming from the reflex mirror. Instead of the solid block of glass of the prism in pentaprism system, here 3 mirrors are used to perform the same task. This is cheaper and lighter, but generally produces a viewfinder image of lower quality and brightness. This optical device is often (more precisely) referred to as ''roof pentamirror'' because of the roof-like ridge.Paul R. Yoder. ''Opto-Mechanical Systems Design'', 2005
page 435

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Understanding ViewfindersPhoto of a Canon 400d pentamirror
{{optics-stub Mirrors Photography equipment

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