Ron Martinsen's Photography Blog: COMPARISON: Variable Neutral Density Filters (Singh-Ray, B+W, Hoya, Tiffen & Bower)
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 (Nano-X Series) : Electronics
Tiffen Variable ND Filter Review | ePHOTOzine
Ultimate Guide: 10 Best Variable ND Filters - Wit and Folly