focusing screen

A focusing screen in optics, often made of ground glass or plastic, is a surface within a camera that displays the image captured by the lens. This allows photographers to frame their shots and control sharpness. While images can be viewed without a screen, a high-power magnifier is needed to precisely determine the plane of maximum sharpness.1

To aid the eye in detecting slight unsharpness, focusing screens frequently include aids like split-image wedges or microprism areas. A split-image wedge divides an out-of-focus image into two displaced halves; when the lens is focused correctly, these halves merge into a continuous image. Microprisms create a ragged, broken-up image that sharpens abruptly upon correct focus. Some screens also use a Fresnel design to ensure the image is evenly bright.1

Cameras without screens often rely on a distance scale or an optical rangefinder. Rangefinders use a viewfinder and a mirror to superimpose two images of the subject, which are aligned when the lens is correctly focused. Modern cameras may also incorporate autofocus systems that use image analysis, infrared rays, or sonar to automatically set the focus.12