Focusing screen

A focusing screen is a flat translucent material, either a ground glass or fresnel lens, found in a system camera that allows the user of the camera to preview the framed image in a viewfinder. Often, focusing screens are available in variants with different etched markings for various purposes. For instance, "overall matte" focusing screens with no etchings are a popular choice for astrophotography and other low-light situations.

The history of the focusing screen is almost as long as the history of the camera. Some primitive cameras consisted of a box with a board holding the lens in the front and a focusing screen in the back that was replaced by the imaging medium (plate, film holder) before taking the picture.

The most common type of focusing screen in non-autofocus 35 mm SLR cameras is the split screen and microprism ring variation that aids focusing and became standard in the 1980s. The microprism ring breaks up the image unless the lens setting is in focus, the split screen shows part of the image split in two pieces. When both pieces are aligned the setting is in focus. The drawback is that the prisms have considerable light loss, making low-light focusing almost impossible. Compare with focusing mechanism in rangefinder cameras.