Minute glass or plastic structure of multiple prisms set in a viewfinder screen to act as a focusing aid. Breaks up an out-of-focus subject into a shimmer but images a focused subject clearly. Will not work satisfactorily at lens apertures smaller than f5.6.

A small number of prisms located on the focusing screens. The microprisms break down out-of-focus images into small segments and appears fuzzy, allowing focusing on subjects without distinct lines.