Every Hack Photographer Needs a Polaroid Variable Range ND Filter
Amazon.com : Polaroid Optics 52mm Neutral Density Filter [ND 0.6] Compatible w/ All Popular Camera Lens Models : Camera Lens Neutral Density Filters : Electronics
The Polaroid Variable ND-filter psychosis – Macs Knows
Neutral Density Shootout from Dave Dugdale: Which Variable ND Filter Should You Buy?