Microprism

From Camerapedia.org

Viewfinder image with microprism ring in the centre, around a split-image device

A microprism is a focusing aid made of an area very small prisms built in to a focusing screen; this causes a slightly out-of focus image to look much fuzzier than on a plain ground-glass screen. Microprisms are often used in SLRs, often in combination with a split-image device.

Olympus focus screen with microprism & split-image