Why Do DSLR Cameras Use Penta Prism?

Nowadays, people are very familiar with DSLR cameras, which are single-lens reflex digital cameras. The principle of DSLR cameras is to use a single lens for framing and the light is projected onto a reflex mirror, which reflects the image onto a focusing screen and forms an image. The image is then reflected onto a penta prism and projected to the viewfinder through a eyepiece. So why does DSLR cameras use pentaprisms? Let's take a look.

Optical Performance of Penta Prism

Penta prism made of high-quality optical glass have excellent optical performance, with a very high reflection rate (theoretically 100% for the reflection field of pentaprisms), and very little loss of light in the optical path, resulting in a bright and clear viewfinder. However, high-quality pentaprisms are expensive and relatively heavy, increasing the cost and weight of the camera. Therefore, high-end DSLR cameras generally use the penta prism as viewfinder devices, while low-end DSLR cameras use a device called pentamirrors as a substitute for the penta prism.

The reflection inside the prism is not caused by total reflection, because the angle of incident light at reflection is less than the critical angle, which is the minimum angle for total reflection. Therefore, both reflection surfaces need to be coated with reflective mirror surface. The incident and exit surfaces are coated with anti-reflection film to reduce reflection. Although the fifth surface is not used, the angle between the two reflection surfaces is an obtuse angle (an angle greater than a right angle).

Advantages of Penta Prism in DSLR Cameras

The imaging principle of DSLR cameras is pinhole imaging. We know that the image formed by pinhole imaging is upside down. Therefore, DSLR cameras use the penta prism to handle the image formed through the lens, and the image is ultimately reflected back to the viewfinder in the form of mirror reflection, which is called optical viewfinder. The internal structure of DSLR cameras has a reflex mirror and a penta prism for various reflections of light. These are the main components for viewing through the DSLR camera, which are also the most special features of DSLR cameras.

The reflection device used for the viewfinder of a pentaprism camera is designed to correct the image upside down on the focusing screen, making the image seen through the viewfinder completely consistent with the orientation of the observed object, so that the operator can accurately frame and focus. When the shutter is released, the reflex mirror flips up, releasing the light path, and the penta prism is not translucent.

The penta prism of DSLR cameras can reflect the image twice, so the image becomes upright, which reflects the image in the correct reflection form to the viewfinder.