NiSi launches variable ND filter without the dreaded X-effect: Digital Photography Review
Amazon.com : NEEWER 67mm MRC Variable ND Filter ND2-ND400, Neutral Density Adjustable ND Filter (0.3 to 2.7,1 to 9 Stops), Multi Layer Coated Optical Glass, Water Repellent & Scratch Resistant Ultra Slim
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Platinum™ 52mm, 58mm and 67mm Variable Neutral Density (ND) Lens Filter PT-MC400VND67 - Best Buy
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Amazon.com : Polaroid Optics 67mm Multi-Coated Neutral Density Fader Filter – Variable Range ND2-ND2000– Holds Optical Quality – Compatible w/ All 37mm DSLR Cameras : Camera Lens Neutral Density Filters : Electronics
Amazon.com : Polaroid Optics 67mm Neutral Density Filter [ND 0.6] Compatible w/ All Popular Camera Lens Models : Camera Lens Neutral Density Filters : Electronics
Amazon.com : Polaroid Optics 67mm Multi-Coated Neutral Density Fader Filter – Variable Range ND2-ND2000– Holds Optical Quality – Compatible w/ All 37mm DSLR Cameras : Camera Lens Neutral Density Filters : Electronics