Model 666681EFN9. 6.6x6.6" (6x6") size for use with matte box. This 6.6 x 6.6" Tiffen Combination 81EF/ND Filter blends both an amber light-balancing 81EF and a 0.9 neutral density filter. The ND filtration provides a 3-stop exposure reduction for darkening the entire image in order to photograph with a wider aperture or slower shutter speed than normally required.