focusing screen

Definition: Plate of glass or plastics material on which image projected by camera lens can be clearly seen for the purpose of viewing and focusing. * In view cameras, the focusing screen is replaced by the film-holder in such a way that the film lies exactly in the same position as the screen, to maintain focus. * In reflex cameras the distance from the focusing screen to the lens is exactly the same as the distance from the lens to the film. * Also, but less commonly, known as viewing screen.
Sophisticated focus
The modern focusing screens is a highly complicated optical device, incorporating e.g. laser-cut microprisms or fresnel lenses to improve the brightness and contrast of the image, as well as aids for manual focusing such as split-prisms, or components such as beam-splitters for exposure measurement or light-piping for projecting information.

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