On the view camera, this ground glass is placed where the film will ultimately go, and is replaced by a sheet of film once focus is correct. Twin lens reflex cameras use two lenses that are mechanically linked, one for focusing and the other to take the photograph. Single lens reflex cameras, meanwhile, use the same objective lens for both purposes, with a mirror to direct the light to either the focusing screen or the film.
View cameras utilize ground glass for focus verification before replacing it with film. Twin lens reflex cameras use two lenses that are mechanically linked for focusing and photography. Single lens reflex cameras employ a single lens for both focusing and capturing the image, using a mirror to direct light to either the focusing screen or the film.