pentaprism

A pentaprism is a five-faced optical prism used in the viewfinder system of many single-lens reflex cameras to present an upright, correctly oriented image to the photographer. Light from the lens is reflected by the reflex mirror upward to the focusing screen; from there the image enters the pentaprism, which uses internal reflections to reorient the image and send it to the eyepiece. The result is an image that is upright and laterally correct, making composition and focusing intuitive.

Construction and operation are based on the properties of total internal reflection within a solid glass block. The pentaprism is positioned between the focusing screen and the viewfinder, and its internal faces are precisely polished to ensure accurate image rotation and minimal light loss. In typical designs, the prism performs two internal reflections to convert the inverted, left-right reversed image into a properly oriented view.

In practice, some cameras use a pentamirror—an arrangement of mirrors that mimics the function of a pentaprism but is lighter and cheaper to produce. Pentaprisms generally provide higher brightness and edge-to-edge sharpness in the viewfinder, at the cost of greater weight and expense. Modern DSLRs and film SLRs may employ either a pentaprism or a pentamirror, depending on design priorities such as brightness, size, and cost.

Historically, the pentaprism became standard in many higher-end SLRs to improve viewing experience, while entry-level bodies commonly used pentamirrors. The basic principle also informs other viewfinder designs, including some mirrorless adaptations that rely on different optical paths.

In summary, the pentaprism is a key component that delivers a correctly oriented view through the camera’s viewfinder by reconstituting the image reflected from the lens system.