I think it really depends on how long a long exposure is; or to put it anothe way, how fluid you want the water to look in your photos? Personally, I wouldn't go with too dark of an ND filter because you can over do the effect, in my opinion. That said, I wouldn't waste my money and get a (.3) filter has it wouldn't have a hugely significant effect. I own a B&W .4X ND filter, and I'm glad I've got it in case I need it. It's dark enough to get my shutter speed down (when doing wider-apertured portraits outside) yet not too strong so that I can't still light the subject with a few strobes. If I were just doing landscapes, a B&W 1.8 ND might do the trick nicely. Take a look at the reviews.


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