Abbe–Koenig prism

The prism is named after Ernst Abbe and Albert Koenig.

Light enters one face at normal incidence, is internally reflected from a 30° sloped face, and then is reflected from a "roof" (German Dach) section (consisting of two faces meeting at a 90° angle) at the bottom of the prism.

The light is then reflected from the opposite 30° face and exits, again at normal incidence.

Unlike the more common double Porro prism configuration, the Abbe–Koenig prism does not displace the output beam from the input beam, making it advantageous in some instruments.

Abbe–Koenig prism and other roof prism binoculars benefit from phase-correction coatings to minimize these problems and substantially improve resolution and contrast.

A diagram of an Abbe–Koenig prism. The prism is shaped like a rectangle with triangular sections removed from the top (reaching from the upper corners to the center of the prism) and lower corners.
In this image, the bottom left and right corners of the prism are not needed and have been cut off to reduce weight.
Binoculars diagram showing an Abbe–Koenig prism