What is the difference between roof prism and porro prism binoculars?

Roof prisms and porro prisms are two major types of prisms used in binoculars.

Binoculars rely on pairs of prisms to fold and erect the image in each tube assembly (binocular half). Doubled roof prisms are small and light enough that a pair of binoculars designed around them can have short, straight, and lightweight tubes. Thus, they are very compact and are the better choice when weight and size are key considerations. Porro prisms and other types of prisms are bulkier and, when paired, result in a tube design that bends out at a side angle. Porro prism binoculars have the classic bulged tube designs and are larger than roof prism pairs.


The type of glass also affects prism optical properties. Major modern glasses used for binocular prisms are BK-7 and BaK-4. BK-7 is a borosilicate glass and is standard in many models of binoculars.  BaK-4 is a higher density and higher refractive index glass that virtually eliminates internal light scattering, producing sharp, well-defined images.


Updated 10/21/13