The prisms inside binoculars turn what would otherwise be an upside-down image right-side up. There are two main types of prism systems used in modern binoculars: porro prisms and roof prisms.
Porro-Prism Binoculars
Each barrel of a porro-prism binocular contains two right-angle prisms. They are offset from each other, which requires that the objective lenses be spaced farther apart than the eyepieces. Thus, porro-prism binoculars are bulkier than their roof-prism counterparts. Optically, however, porro-prism binoculars usually perform better, because the prism design requires less strict tolerances. That makes them easier to manufacture, so they cost less. Also, porro prism binoculars yield a more stereoscopic, or three-dimensional, image.
Roof-Prism Binoculars
The prisms in a roof-prism binocular overlap closely, allowing the objective lenses to line up directly with the eyepieces. This results in a more streamlined, compact, and often more lightweight binocular than equivalent porro-prism models. But roof-prism binoculars are also more difficult to manufacture, so they cost more. Roof prisms lose slightly more light to reflection than porro prisms - a disadvantage for astronomical use but not a concern for daytime terrestrial viewing. Well-made roof-prism binoculars can provide optical performance nearly equal to, but not better than, porro-prism binoculars.
Prism Glass
Most optical prisms are made from either BK-7 (borosilicate) glass or BAK-4 (barium crown) glass. BAK-4 is the higher-quality glass and yields brighter images and better edge sharpness. It is also more expensive.
