Porro–Abbe prism

It is a variant of the more common double Porro prism configuration.

[1] It is made from a piece of glass shaped like four right-angled reflecting prisms joined face-to-face in a twisted fashion.

Light enters one flat face, is internally reflected four times (incident at an angle of 45º) from the sloping faces of the prism, and exits the second flat face offset from, but in the same direction as the entrance beam.

The Porro–Abbe system reduces the lateral beam axis offset by 23% compared to a traditional double Porro prism system in binoculars.

A single half of the assembly is sometimes also called a Porro–Abbe prism.