This is the world's first variable graduated ND filter: Digital Photography Review
Amazon.com : Polaroid Optics 43mm Multi-Coated Variable Range [ND3, ND6, ND9, ND16, ND32, ND400] Neutral Density Fader Filter ND2-ND400- Compatible w/ All Popular Camera Lens Models : Camera Lens Neutral Density Filters :
How to fit a Neutral Density filter with your Polaroid camera - YouTube
How to make a film pack ND filter for your Polaroid SX-70 – Angie Muldowney
Handmade ND Filter for SX-70 Cameras Use Polaroid 600 Film - Etsy
Gear Review: Polaroid Variable ND Filter | LG Visuals ⎟ Lewis Gething Visuals Blog
Polaroid 77mm Neutral Density Fader Filter PLFILFDND77 B&H Photo
Gear Review: Polaroid Variable ND Filter | LG Visuals ⎟ Lewis Gething Visuals Blog
Polaroid Optics 37mm Neutral Density Filter [ND 0.6] Compatible w/ All Popular Camera Lens Models - Walmart.com
Gear Review: Polaroid Variable ND Filter | LG Visuals ⎟ Lewis Gething Visuals Blog