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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 

The benefit of this adapter is that it’s suitable for Pentax DA lenses that don’t have an aperture ring. The silver ring on the adapter controls the aperture linkage on the back of the lens. Although a lens might typically have a aperture range of f2.8 to f22, the adapter only has 5 stops, and the real value of the aperture is guesswork, however a mft camera will have no trouble gaining the correct exposure. Lenses with their own aperture ring can still be used on this adapter, you just need to see the adapter to ‘open’. 

No, it’s only for Leica m mount. You will need a different version for l mount. 

Yes, it is indeed Nikon f mount.