Focusing screen

A focusing screen is a flat translucent material, either a ground glass or Fresnel lens, found in a system camera that allows the user of the camera to preview the framed image in a viewfinder.

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 drawback is that the prisms have considerable light loss, making low-light focusing almost impossible.

Some models have markings etched in them to denote the areas on which the camera focuses or calculates the exposure from.

Many newer midrange and professional digital SLR cameras possess a plain screen with a monochromatic LCD overlay that reveals focus points as needed.