|Focussing Screen on a |
Konishi Field Camera
image by Dirk HR Spennemann (Image rights)
A ground glass is an element of many camera viewfinder systems, for example reflex and view cameras. A clear screen corresponding to the size of the film or sensor is given a finely-etched, frosted surface on one side. The camera lens focuses its image onto this surface, and the photographer views this from the opposite side to evaluate focus and framing. Because of this use, the term focus screen is also used.
Historically, the viewing screen would have been literally a piece of glass, ground with a fine abrasive. In contemporary cameras, molded plastics are used instead. The molding process permits the ground glass to include focusing aids, such as tiny microprisms which "shatter" an out-of-focus image; or a prismatic split-image spot emulating rangefinder focusing. Many high-end film SLRs had interchangeable focus screens, allowing a choice of focus aids and framing lines. A ground glass often includes a fresnel lens which helps redirect light towards the photographers eye, yielding better viewfinder brightness. Depending on the design of the camera, the photographer may view the ground glass directly, through a waist-level hood, a magnifying "chimney," or an eye-level pentaprism.
Large format cameras (cameras which use film 4x5 inches or larger) such as view cameras often have no separate viewfinder system at all, relying entirely on a ground glass at the focal plane to focus and compose the image. Before a photograph is taken, the ground glass is attached to the back of the camera, and the lens opened to its widest aperture. This projects the scene onto the ground glass, upside down and backwards. The photographer focuses and composes using this projected image, sometimes with the aid of a magnifying glass or loupe. In order to see the image better, a dark cloth is used to block out light, the origin of the image of an old-time photographer with his head stuck under a large black cloth.