The pentaprism finely ground from high-quality optical glass has excellent optical performance and high reflectivity (theoretically, the reflective field of view of the pentaprism is 100%), and the light loss in the optical path is very small, so the viewfinder field is bright and clear. However, the high-quality pentaprism is expensive and heavy, which increases the cost and weight of the whole camera. Therefore, the general high-end SLR camera uses the pentaprism as the viewfinder, while the general low-end SLR camera uses a device called pentahedral mirror to replace the pentaprism.
The reflection inside the prism is not caused by total reflection, because the angle of the incident light at the time of reflection is less than the critical angle, that is, the minimum angle of total reflection, so both reflecting surfaces should be plated as reflecting mirrors; The incident and outgoing surfaces should be coated with anti reflection film to reduce reflection. Although the fifth surface is not used, the included angle with the two reflecting surfaces is an obtuse angle (greater than the internal angle of a right angle).
