A pentaprism is a multi-sided glass prism housed in the top "roof" section of a single-lens reflex (SLR) camera. It allows the photographer to see exactly what the lens sees through the optical viewfinder. Light entering through the lens is first reflected upward by the reflex mirror, then directed across the pentaprism, which corrects the image orientation so it appears right-side-up and laterally correct to the eye.
Pentaprisms are essential in traditional SLR cameras, providing a bright, correctly oriented view for accurate framing and focusing. While digital cameras sometimes use electronic viewfinders instead, the pentaprism remains a key feature in many professional-grade SLR cameras for precise optical viewing.