A device, apparatus, or control used for bringing an image into focus, typically consisting of a mechanism that allows the position of one or…

Photography. An instrument used as a visual aid to focusing a non-reflex camera, usually internal and coupled to the lens. Also: a camera so equipped.

a device consisting of a prismatic arrangement of mirrors (see quots.); so beam-splitting adj.

Photography. A lens attachment which produces a radiating pattern of rays around the image of a source of light; (also) the effect so produced.