Pentamirror vs. Pentaprism — What's the Difference?

A pentamirror is an optical device used in the viewfinder systems of various single-lens reflex cameras instead of the pentaprism. It is used to reverse again the laterally reversed image coming from the reflex mirror.Instead of the solid block of glass of the prism in pentaprism system, here 3 mirrors are used to perform the same task.

A pentaprism is a five-sided reflecting prism used to deviate a beam of light by a constant 90°, even if the entry beam is not at 90° to the prism. The beam reflects inside the prism twice, allowing the transmission of an image through a right angle without inverting it (that is, without changing the image's handedness) as an ordinary right-angle prism or mirror would.

(photography) A system of three (not five) mirrors used as a cheaper substitute to a pentaprism in the viewfinders of some single-lens reflex cameras

(photography) A five-sided prism used in the viewfinder of most single-lens reflex cameras