Rotary shutters are semicircular discs that spin in front of the film gate, alternately allowing light from the lens to strike the film, or blocking it. When blocking the light, the exposed portion of the film is advanced, and another, pristine frame is moved into position. The spinning disc then exposes the new frame of film. The nature of light-sensitive film requires a shutter to block the light and prevent "smearing" of the image as the film is advanced through the camera. Digital sensors do not require a rotary shutter, although some high-end digital cinema cameras do have them. Some rotary shutters utilize mirrors (or mirrored surfaces) so that when the shutter is in its "closed" position, light from the lens is redirected to a viewing system, allowing the camera operator to view, frame, and focus the image, exactly as the film sees it.