77mm ND1000 (10 Stop) Filter Nano-Xcel Series - Multi-Coated Optical Glass, Fixed Neutral Density Lens Filter

SKU: KF01.977

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Yes, there are threads on both sides (I just tried), which can be useful for adding a polarizer. However, you can just as easily have the K&F ND1000 filter stacked onto the inner filter. 
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Yes, I bought this filter (82mm) specifically for my Sony 24-70mm G-master. Naturally, not all Sony lenses have this diameter filter thread. 
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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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Filter size is filter size. Internal thread measurement for any focal length as long as the outside filter diameter is 58mm.