Penta prism is one of the beam deflectors that deviates light at a fixed angle (90°). It serves two main purposes: regardless of the angle of incidence on the first surface, the exiting light deflects the incident beam by a specific angle (90°), and unlike right angle prisms, it produces an image that is neither rotated nor mirror-reflected. This prism is used in camera viewfinders and is particularly suitable for image observation systems or measuring instruments.

The penta prism cannot achieve total internal reflection based on the critical angle principle. Therefore, its two reflective surfaces are typically coated with an aluminum coating and black painted, while the entrance and exit surfaces are coated with a single-layer MgF₂ anti-reflection coating.