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. Often, focusing screens are available in variants with different etched markings for various purposes. For instance, when photographing landscapes, a focusing screen with a grid allows the photographer to keep the horizon straight. Modern mirrorless cameras do not need a focusing screen since they display what the image sensor sees on a flat-panel display or electronic viewfinder.