Square Filter ND64(6 Stops) 100x100mm Square Filter with Protective Frame, Waterproof ND Neutral Density Filter Light Reduction Filter made of HD Optical Glass, Nano-Xcel Pro Series

SKU: SKU.1873

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There are coatings on both sides, and the middle is transparent. 
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They fit all filter holders that don't have to have an additional seal glued to the filters. I use it myself with the Rollei Mark II filter holder and the Haida M10.