Microprism focusing screen. Light micrograph of the microprism area of a camera's focusing screen. The centre circle of the screen contains a region filled with tiny, pyramidical prisms etched on the surface. If the camera lens is not correctly focused, the user sees a crazed image in the centre of the viewfinder. When the correct focus is achieved, the centre of the viewfinder appears to clear, showing a sharp image. Outside of this area, the screen is etched as a fresnel lens. This type of focussing screen is also used in photomicrography. Magnification x4 at 6x7cm size. Magnification: x6.5 at 5x4 inch size.