A roof prism is in general any kind of reflective optical prism containing a section where two faces meet at a 90 degree angle. Reflection from the two faces returns and image that is flipped laterally across the axis where the faces meet.
Roof prism binoculars have a straight profile, with the eyepiece situated directly behind the front lens. Roof Prism binoculars are 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.
Roof prisms offer a newer design. They are smaller and more compact than Porro prisms, and actually resemble a small house with a peaked roof. This allows them to be fitted into smaller housings, which makes for more compact binoculars or spotting scopes.
Standard roof prism models of at least average quality are a good choice for birding, as long as they have all the other features of a good birding glass. But if you demand the best in resolution and sharpness, keep in mind that these roof prisms can only do so much without phase correction. A birder whose first concern is resolution must therefore pay the price for a PC roof prism, or opt for a good Porro prism.
Roof Prism models will deliver more durability but you will need to spend more money to get good optical quality in this design.
Roof prism are not as bright as Porro prisms because they employ mirror surfaces that lose some light. In addition, alignment is often more critical than in Porro prisms; an alignment error a fraction of the width of a human hair will degrade the image in a roof prism. Securing them in binoculars is much more of a challenge. And lastly, images are split and then rejoined slightly out of phase when they pass through such a prism. This produces an image of slightly less resolution than an equal quality Porro prism.
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