focusing screen

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.

Using a focusing screen is a matter of practise. Most focusing screens use a combination of three different focus aids.

The first and most obvious is a split image telemeter, this looks like a circle split in two. To use it, simply adjust the focus until the two halfs of the image line up. This is extremely easy where the image has lines running perpendicular to the split in the telemeter, but if you are trying to focus on a random pattern (like a tree far away) it can be tricky.

In this case, shift your eye to the ring surrounding the telemeter, this is a microprism, and will show a patterned image if it is out of focus, just adjust the focus until the microprism becomes clear.

The final aid is the ground glass of the screen itself, use this to judge by eye when the image looks sharp and in focus.

Diagram of a typical focusing screen:

 _______________________________
|                               |
|                               |
|                               |
|      gg    ______             |
|           /m __  \            |
|           | |__| |            |
|           | |__| |            |
|           \______/            |
|                               |
|                               |
|_______________________________|