Binocular PrismsA prism is a transparent object that admits light but causes the rays to get bent or shifted. These let you see a correctly oriented image when you look through a pair of binoculars. Prism binoculars employ prisms in the optical system to rectify an inverted image projected by the objective. The prism also shortens the optical path, resulting in a compact body. The function of a prism in a binocular or spotting scope is to provide a properly oriented image. Prisms are expensive, but without them a binocular would always produce upside-down, reversed images, challenging even the most skilled birder. There are two types of prism binoculars in common use, Porro prisms and roof prisms.
Porro prism binoculars have the classic “dog leg” shape with the eyepiece offset to the one side with respect to the objective or front lens. These are like a great horned owl, wide, bulky, and stout. They provide a big, steady platform for observing and offer a nice hand-filling package, especially for people with large hands. Porro prisms tend to give a sharper image than standard roof prisms.
Roof prism binoculars have a straight profile, with the eyepiece situated directly behind the front lens. These are more like a harrier trim, narrow, and straight. This allows them to be held with arms tucked close to the body for maximum steadiness. The narrow profile of a roof prism also places focusing wheels conveniently under the fingers, which enhances focusing speed and comfort, especially for people with smaller hands. This type of binoculars are appreciated for their streamlined shape and durability.
