pentaprisms

Pentaprisms, or pentaprisms, are five-sided prisms used in the viewfinder systems of many single-lens reflex (SLR) cameras to produce an upright, correctly oriented image for the photographer. They correct both the upside-down and left-right reversal that occurs as light travels from the lens through the reflex mirror.

In an SLR, light from the scene is reflected by the primary mirror up into the viewing path. The pentaprism intercepts this light and, through internal reflections, redirects it to the viewfinder with the image oriented as seen by the eye. The result is a right-side-up image with the correct left-right orientation, allowing intuitive framing and focus.

Most traditional pentaprisms are solid glass blocks with precisely coated interior surfaces to achieve high reflectivity across the light spectrum. The five-faced shape is designed to create the necessary two 90-degree turns to deliver the upright image. Modern variants include pentamirs, or pentamirror assemblies, which use three mirrors instead of a solid prism. Pentamirrors are cheaper and lighter but generally deliver a slightly dimmer view due to extra air-glass interfaces.

Usage and limitations vary by camera design. Pentaprisms were standard in many film-era SLRs and remain common in many DSLR models, while some cameras substitute electronic viewfinders. In mirrorless cameras, traditional pentaprisms are not used, as the viewfinder path is electronic.

Overall, pentaprisms provide a bright, reliable means of viewing through the lens with correct orientation, contributing to accurate composition and focus in many SLR systems.