Porroprisms

A porroprism is a type of optical prism that deviates a beam of light by 180 degrees while reinverting the image. It consists of two right-angled prisms joined together. The light enters one prism, reflects internally twice, and then exits the second prism. The two internal reflections cause a 180-degree deviation of the light path. Additionally, these reflections cause both the image and its orientation to be inverted. This makes porroprisms useful in applications where an erect and laterally reversed image is desired, such as in binoculars.

The primary advantage of the porroprism design is its ability to produce a compact optical system with a long focal length. The zig-zag path of the light within the prism effectively doubles the distance the light travels compared to a single lens system with the same physical dimensions. This allows for higher magnification and a wider field of view. Porroprisms are commonly found in traditional binoculars, often characterized by their stepped, offset objective lenses. Despite the advent of newer prism designs like roof prisms, porroprisms remain a popular choice due to their cost-effectiveness and optical performance.