a camera in which the image appears on a ground-glass viewer (focusing screen) after being reflected by a mirror or after passing through a prism or semitransparent glass; in one type (single-lens reflex camera) light passes through the same lens to both the ground glass and the film, while in another type (twin-lens reflex camera) light passes through one lens (viewing lens) to the ground glass and through a second lens (taking lens) to the film, the lenses being mechanically coupled for focusing.
British Dictionary definitions for focusing screen
a camera in which the image is composed and focused on a large ground-glass viewfinder screen. In a single-lens reflex the light enters through the camera lens and falls on the film when the viewfinder mirror is retracted. In a twin-lens reflex the light enters through a separate lens and is deflected onto the viewfinder screen