Cokin 67mm NUANCES Variable ND Filter (5 to 10-Stop) CNV32-67
Amazon.com : NiSi 100X100mm Square Neutral Density Filter,IR ND64(1.8), ND 6 Stops 100mm System Optical Glass for 52mm,55mm,58mm,62mm,67mm,72mm,77mm,82mm Lens,Compatible with Lee Cokin Hitech Singh-Ray : Electronics