Prism and prism system
The soul of binoculars is a set of prisms. The function of prism is to change the inverted image back to the upright image. Prism system can be divided into Porro (Paul prism) system and roof (roof prism) system, according to the material can be divided into bak4 material and BK7 material.
The two most common prism systems for binoculars are the Porro and roof prism systems. The distance between the two objective lenses of Paul's binoculars is larger than that of the eyepiece. The optical quality of the prism near the ridge is cheaper than that of the prism at the roof. Roof binoculars, small size, can make the objective and eyepiece in a straight line, commonly used in compact binoculars.