Parallax: Is an apparent displacement or difference in apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination.

Pentaprism: Is a five sided reflecting prism used to deviate a beam of light by 90 degrees.

pellicle mirror: Is an ultra thin, ultra lightweight semi transperant mirror employed in the light path of an optical instrument, spliting the light beam into two seperart beams, both of reduced light intensity.

Beam splitter: Is an optical device that splits a beam of light in two

The inside of a camera: