Pentaprism and pentamirror are used in the viewfinder systems of single-lens reflex cameras.
A pentaprism is a five-sided reflecting prism used to deviate a beam of light by 90°.
Pentamirror is the optical device used in the viewfinder systems of cheaper single-lens reflex cameras instead of the pentaprism. It is used to reverse again the upside-down and laterally reversed image coming from the reflex mirror.
Instead of the solid block of glass of the prism in pentaprism system, here 3 simple mirrors are used to perform the same task. This is cheaper and lighter, but generally produces a viewfinder image of lower quality and brightness.
D70 and upper
D60 and lower, D5000
D60 及以下, D5000