The computar CBF-1 Camera Back Focusing Neutral Density Filter with Case is used primarily to make the proper back focus adjustments when installing motorized zoom lenses. To assure that a zoom lens stays in focus throughout the zoom range, the camera must be back focused when lens iris is wide open. The camera is placed in front of the lens to keep the auto-iris open during adjustment. The filter is 4 x 4" (101.6 x 101.6mm) square and made of glass (not plastic) and has an ND rating of 3.0. The CBF-1 filter is an essential tool for the professional installer.