I use Lee's Big Stopper. 10 stop ND filter. It gives a very blue color cast but thanks to RAW processing I'm able to adjust the colors and white balance easily. Has taken a lot of practice to get it down to a science.
I'm going to have to get one of those. They're a screw in filter are they? Guess I'd get a wide one and step down rings if it is.
What lens and setting please? Just curious ..... thanks