Pentaprism: Deflecting Light Beams Precisely by 90 Degrees

Definition of a Pentaprism

A pentaprism consists of two refractive surfaces at 90° to each other and two reflective surfaces at a 45° angle. A pentaprism is a type of optical prism with five reflective surfaces. Its design is such that when light enters through a specific entrance surface, it undergoes multiple reflections within the prism and finally exits through another specific exit surface at a 90° angle. This process preserves the orientation of the image both horizontally and vertically, without altering the image's handedness.

Design of a Pentaprism

The design of a pentaprism is very ingenious, utilizing the principle of total internal reflection to maintain the directionality of light as it undergoes multiple reflections inside the prism. Due to this characteristic, pentaprisms are often used in situations where it is necessary to change the direction of light without altering the image's characteristics, such as in camera viewfinders, telescopes, microscopes, and other optical instruments.

Application of Pentaprism in SLR Cameras

The pentaprism is a reflective device used in SLR camera viewfinders. Its function is to correct the inverted image on the focusing screen, making the image seen through the viewfinder match the orientation of the actual scene, thus allowing the operator to properly frame and focus the image.

When the camera is not being operated or is in a focusing state, light enters the camera through the lens aperture. At this time, the reflex mirror is in a lowered position, blocking the photosensitive element and reflecting the light into the pentaprism. Due to the special angles of the pentaprism, after multiple reflections, the light exits through the camera's eyepiece, and the aperture's small hole image is reoriented to an upright position. This explains why people can see an upright image through the eyepiece that matches the captured scene.

Optical Performance of Pentaprism

A penta prism made from high-quality optical glass with precise grinding has excellent optical performance, with very high reflectivity and minimal light loss, resulting in a bright and clear viewfinder image. However, high-quality pentaprisms are expensive and relatively heavy, which increases the overall cost and weight of the camera. Therefore, pentaprisms are typically used in high-end SLR cameras, while lower-end SLR cameras often use a device called a pentamirror to replace the pentaprism.