A focusing screen is a flat translucent material, either a

ground glass Ground glass is glass whose surface has been ground to produce a flat but rough (matte) finish, in which the glass is in small sharp fragments. Ground glass surfaces have many applications, ranging from ornamentation on windows and table glassw ...

Fresnel lens A Fresnel lens ( ; ; or ) is a type of composite compact lens developed by the French physicist Augustin-Jean Fresnel (1788–1827) for use in lighthouses. It has been called "the invention that saved a million ships." The design allows the c ...

, found in a

system camera A system camera or camera body is a camera with interchangeable components that constitutes the core of a system. Early representatives include Leica I Schraubgewinde (1930), Exakta (1936) and the Nikon F (1959). System cameras are often sin ...

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

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 ...

. 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.

Overview

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 An autofocus (or AF) optical system uses a sensor, a control system and a motor to focus on an automatically or manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system ...

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

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 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 An autofocus (or AF) optical system uses a sensor, a control system and a motor to focus on an automatically or manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system ...

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. Image:Fresnellinse.jpg, Standard focusing screen of the Nikon F (1959–1971) Image:Fresnellinse_mattscheibe.jpg, Other side of the same focusing screen Image:Focus-screen-1.jpg, Split screen and microprism - out of focus Image:Focus-screen-2.jpg, Split screen and microprism - in focus Image:Mattscheibe mit Klarglasfleck und Schnittbildindikator-2731.jpg, Focusing screen with split screen indicator ( Edixa Reflex Ba, ≈1965)

Overview

See also