I can't seem to find any 10-stop ND filters for 82mm threads. Does anyone happen to know off the top of their head who might make or sell these? It doesn't have to be exactly 10 stops, but something that can give me a really long exposure. Or, alternatively, can I just stack two 4 stop ND filters to make an 8 stop ND effect, or does the math not work that way? Thanks!