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i did it with 2 tiffen cpl and it worked well (i got between 2 and 10 stops) much sharper than my $10 fotga vnd. at first i tried to unscrew a cpl with a screwdriver but it was impossible to do - then i used 2 nails in a piece of wood (the sharp part of the 2 nails one inch out of wood so you can easily bend them to adjust the distance between the 2 nails. but i had a problem with b&w and heliopan : they don't have the 2 opposite ridges as in the tiffen cpl, does someone knows how to unscrew these kind of brass filters ? thanks (male to male ring adapter is a simple idea for a few bucks but it add some thickness to the final vnd filter so i'd rather revert one cpl)