Tiffen 77mm Variable Neutral Density Filter 77VND B&H Photo Video
Amazon.com : NiSi 77mm Swift True Color ND-Vario - Variable Neutral Density Filter 1-5 Stops (ND2-ND32) - Rotating Adjustable ND, Compatible with NiSi Swift System Filters - Optical Glass, Waterproof Nano Coating : Electronics