King of converting
That looks a little extreme ... are you trying to avoid short exposures (and why?). Also: have you tried using two polarizers (first linear, second circular) instead?
They also come in pairs as "variable ND" filters. They can get quite dark...
@Mr_Teatime it's just a load of steprings because i didn't find a single one for this particular step - not handy but it works - the nd-filter is variable :)
One of the first Mastodon instances, there is no specific topic we're into, just enjoy your time!