FFocusing Screen
An element between the mirror and pentaprism or pentamirror in a single-lens-reflex camera. The mirror reflects the image from the lens upward onto the screen. The areas of the image that are in focus are sharply defined on the screen, while the areas that are out of focus appear blurred. The photographer views the image passing through the screen in the viewfinder window. Some Nikon SLRs and D-SLRs have interchangeable focusing screens.