The CBF-1 Neutral Density Filter 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 backfocused when lens iris is wide open. The CBF-1 is placed in front of the lens to keep the auto-iris open during adjustment. The filter is 4" x 4" 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. Protective case included.