|Microprism collar surrounding split image spot
image by Voxphoto (Image rights)
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 "sparkle" or "shatter" compared to the surrounding plain ground-glass screen. Microprisms are often used in SLRs, often in combination with a split-image device.
|Removable Olympus focusing screen;
microprism ring surrounds split-image center
image by Tim Williams (Image rights)