A variable diameter mounting attachment joins an accessory, such as a matte box, to a lens of a camera, such as a video camera, digital camera or broadcast camera, while blocking undesired light from reaching the lens. The attachment incorporates two synchronized mechanisms activated together by rotation of an outer gear ring: (1) a series of jaws to clench an outer edge of the lens housing, and (2) a multiple blade iris diaphragm to block undesired light from entering the lens. A lock lever clamps the outer gear ring in fixed position.