The ND values of the three filters are different. In order to avoid the phenomenon of suitable exposure for wide-angle cameras and medium telephoto cameras and underexposure for telephoto cameras, the light reduction values of the three lenses are slightly different. For example: Wide angle ND8, medium telephoto ND8, telephoto for ND0 Wide angle ND16, medium telephoto ND16, telephoto ND8 Wide angle ND32, medium telephoto ND32, telephoto ND16 Wide angle ND64, medium telephoto ND64, telephoto ND3 ND&PL filters also have the same ND value distribution as ND filters 

If it is a magnetic filter from K&F CONCEPT, it can be used as long as the caliber is the same. It is not possible if it is a threaded or other brand filter. 

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yes. 

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.