The role of discount penta prism

The discount penta prism is a reflective device used for framing of the SLR camera. Its function is to correct the upside-down image on the focusing screen, so that the image seen in the framing is exactly the same as the scene directly seen, so that the operator can correctly framing and focus. When the shutter is opened, the mirror is turned up to let out the light path, and the discount penta prism is opaque.
The discount penta prism is usually a solid piece of glass that has been cut and ground, and then coated with reflective material on the outside (except the two surfaces that are connected to the focusing screen and the viewfinder eyepiece) to form a specular reflection inside.

discount penta prism
The discount penta prism made of high-quality optical glass has excellent optical performance, very high reflectivity, and very little light loss in the optical path, so the viewing field is bright and clear. However, high-quality discount penta prism are expensive and heavy, adding to the cost and weight of the entire camera. Therefore, generally high-end SLR cameras use a discount penta prism as a viewfinder, while generally low-end SLR cameras use a device called a pentamirror instead of a discount penta prism.
The reflection inside the discount penta prism is not caused by total reflection, because the angle of the incident light during reflection is less than the critical angle, which is the minimum angle of total reflection, so both reflective surfaces should be coated as mirror surfaces; the incident and exit surfaces should be Anti-reflective coating is applied to reduce reflections. Although the fifth surface is not used, the angle between it and the two reflecting surfaces is an obtuse angle.



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