| 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