Binocular Prism System
Binocular Prism System – The function of a binocular prism system is to reduce the size needed to provide focal length and turn the resulting upside-down image right-side-up. Most optical prisms are made from borosilicate (BK-7) glass or barium crown (BaK-4) glass. BaK-4 is the higher quality glass yielding brighter images and high edge sharpness. There are two types of binocular prism system: roof and porro prisms.
The roof prism is considered the ideal binocular prism system for general use optic systems for bird watching, wildlife viewing and at sporting events. In a roof prism binocular, the prisms overlap closely allowing the objective lens to line up directly with the eyepiece. This alignment of this binocular prism system results in slim and streamlined-shaped binoculars with the lenses and prisms in a more or less straight line with your eyes. However, this binocular prism system configuration results in a reduction in three dimensional effect in close images. Roof prism binoculars require much more precise (and therefore more expensive) optical fabrication and collimation than porro prism binoculars. Because of the accuracy required, roof prisms are usually laser collimated and are rigidly mounted on alignment plates prior to being installed in the binoculars, allowing virtually permanent collimation. The design of most roof prisms requires that one surface of the prism be an aluminized mirror. This means that the binocular prism system utilizing a roof prism typically will not be as bright as similar-aperture porro prism models, due to the 12% light loss typical from using a mirror. During daylight observing with this binocular prism system, this small light loss is rarely visible. Most observers feel that the convenience of the roof prism system, which utilizes a compact size and lower weight, more than makes up for this light loss. A premium binocular prism system that utlizes roof prisms that are fully multicoated optics and use high-reflectivity silver mirrors can be as bright or brighter than similar-aperture porro prism models, although for a much higher price.
The porro prism is also considered the perfect binocular prism system for general use optics systems for bird watching, wildlife viewing, and at sporting events. In a porro prism binocular, the front lens is spaced wider from the eyepiece which results in a more zigzagged light path. With this binocular prism system, the longer light path allows for greater magnifications and the ability to use larger objective lenses for better performance in low light conditions. In this binocular prism system, the objective lenses can also be spaced further apart than your eyes which creates a more three-dimensional image when used for close in viewing (nearer than 25 to 30 feet). In addition to this enhanced sense of depth perception, this binocular prism system provides a wider field-of-view. Due to the simplicity in design of this binocular prism system, some of the best values can be with porro prism binoculars.
When selecting the appropriate binocular prism system, determine the primary use for that binocular and then consider how its optical characteristics will directly affect its flexibility for use in other applications. To get the biggest bang for your buck, do not compromise on the quality of the binocular prism system.