part of an SLR camera (contrast with the ground glass in a view camera).
The focusing screen is mounted underneath the pentaprism, and above the mirror. The image that the photographer sees on the viewfinder is formed on the focusing screen.
Focusing screens can be plain, but usually they are decorated with focusing aids (prevalent in older manual SLRs, usually microprism and split image telemeters) and/or reference lines, very useful in architectural photography.
In some SLRs the focusing screen is interchangeable.