Focusing screen
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A focusing screen is a flat translucent material, usually ground glass, found in a system camera that allows the user of the camera to preview the framed image in a viewfinder. Oftentimes, a focusing screen has etched markings that differ from model to model. Nikon makes focusing screens of different types that are categorized using letters of the alphabet and denote etchings of different designs. For instance, the Nikon D Screen has no markings and is referred to as an "overall matte" focusing screen. It is 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. One could say that 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 in non-autofocus 35 mm SLR cameras is the split screen and microprism ring variation that aids focusing that became standard in the 1980's.
The microprism ring blurs 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 a considerable light loss making low light focusing almost impossible.
Professional cameras give the photographer a choice of screens that are, depending on the camera model, more or less easy to replace. For low light situations the screen of choice is plain, for architectural images and very wide angle lenses the choice is one with a grid etched on it to control the perspective distortion, for fast focusing the spit screen is the screen of choice and so on.
Cameras with interchangeable film formats (view cameras, field cameras and some medium format cameras) have also etchings on the screen to show the limits of the films. Most of these cameras have either plain or grid screens because due to the size of the focusing screen the only focusing aid really needed is a magnifying glass.
Autofocus SLR, both digital and film, have a plain screen, some models have markings etched in them to denote the areas on which the camera focuses or calculates the exposition from.
