82mm ND1000 Filter 10 Stops ND, Solid Neutral Density Lens Filter Multi-Coated Optical Glass Neutral Grey ND with Multi-Resistant Coating Nano-Xcel series

SKU: KF01.978

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Yes you can, I just tested this by screwing on a 82mm polarizer filter. But keep in mind: 1) This can easily cause vignetting with wide angle lenses; 2) colour shifts because of camera sensor reciprocity failure as well as any colour casts inherent to the second filter. 3) Slight resolution and contrast losses. 
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Canon has at least 7 different 50mm lenses. You need to look on your lens to see what filter size it takes. It'll be written on the lens at the front, beside this symbol: Ø. 
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No I don't think it will fit any Gopro not unless it's specifically made for it. This size fits a 58mm lens thread and the Gopro has a much smaller size lens. 
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