A focusing screen is a flat translucent material, usually ground glass

Ground glass

Ground glass is glass whose surface has been ground to produce a flat but rough finish.Ground glass surfaces have many applications, ranging from mere ornamentation on windows and table glassware to scientific uses in optics and laboratory glassware....

, found in a system camera

System camera

A system camera is a camera with interchangeable components that constitutes the core of a system. Early representatives include Leica I Schraubgewinde , Exakta and the Nikon F...

 that allows the user of the camera to preview the framed image in a viewfinder

Viewfinder

In photography, a viewfinder is what the photographer looks through to compose, and in many cases to focus, the picture. Most viewfinders are separate, and suffer parallax, while the single-lens reflex camera lets the viewfinder use the main optical system. Viewfinders are used in many cameras of...

. 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. 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 type of focusing screen in non-autofocus

Autofocus

An autofocus optical system uses a sensor, a control system and a motor to focus fully automatic or on a manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system has to be done manually until indication...

 35 mm SLR cameras is the split screen and microprism ring variation that aids focusing and became standard in the 1980s. 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 align

Align

Align may refer to:* An album, Align released by Halifax .* The process of alignment * Align * Asset Point...

ed 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 camera

Rangefinder camera

A rangefinder camera is a camera fitted with a rangefinder: a range-finding focusing mechanism allowing the photographer to measure the subject distance and take photographs that are in sharp focus...

s.

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

Perspective (visual)

Perspective, in context of vision and visual perception, is the way in which objects appear to the eye based on their spatial attributes; or their dimensions and the position of the eye relative to the objects...

 distortion, for fast focusing the split screen is the screen of choice and so on.

Cameras with interchangeable film formats (view cameras, field cameras and some medium format cameras) may have etchings on the focusing 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

Autofocus

An autofocus optical system uses a sensor, a control system and a motor to focus fully automatic or on a manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system has to be done manually until indication...

SLR cameras, both digital and film, usually have a plain screen. Some models have markings etched in them to denote the areas on which the camera focuses or calculates the exposure from. Many of the newest midrange and professional digital SLR cameras possess a plain screen with a monochromatic LCD overlay that reveals focus points as needed.