Amazon.com : K&F Concept 77mm Variable ND Filter ND2-ND32 Camera Lens Filter (1-5 Stops) No X Cross HD Neutral Density Filter with 28 Multi-Layer Coatings Waterproof : Electronics
Best variable ND filter - Camera Jabber
Tiffen Variable ND Filter Review | ePHOTOzine
Amazon.com : Freewell 77mm Threaded Hard Stop Variable ND Filter Standard Day 2 to 5 Stop : Electronics
Kenko-Tokina launches NDX variable neutral density filter: Digital Photography Review
NiSi launches variable ND filter without the dreaded X-effect: Digital Photography Review