Platinum™ 52mm, 58mm and 67mm Variable Neutral Density (ND) Lens Filter PT-MC400VND67 - Best Buy
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52mm Neutral Density Filter - Moondog Labs
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Amazon.com : High Definition 52mm 10-Stop NDX Neutral Density Variable Filter for Olympus M ED 9-18mm f/4.0-5.6 Micro Four Thirds Lens & Olympus M.ZUIKO Digital ED 12-50mm F3.5-6.3 EZ Lens : Electronics