Amazon.com : Heliopan 37mm Neutral Density 4x (0.6) Filter (703736) with specialty Schott glass in floating brass ring : Camera Lens Neutral Density Filters : Electronics
Heliopan ships Vario ND variable filters in sizes from 55 to 82mm: Digital Photography Review
Genuine Heliopan ES 77mm Slim Variable ND Neutral Density Filter (ND 0.3-ND1.8) | #1791674111
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Variable Gray Neutral Density Filter – Heliopan Filters