Everyone tells you – no filters on the fisheye. But actually, the inbuilt hood has a 58mm filter thread on the inside. Filters – yes you can. Now I am a little nervous of the front element at the best of times. But the threat of crushing it with a filter is too much for me, so I have not put a 58mm filter on it (although others have: http://www.pentaxforums.com/forums/pentax-slr-lens-discussion/54026-da-10-17mm-fisheye-filter-possible.html ). I have been brave enough to use a 58-77 stepping ring and a 77mm filter. The stepping filter brings it just forward enough for me to be comfortable. Obviously there is a bit of vignetting. Anything under 14mm needs a bit of cropping or corner cloning, but I can live with that. So now I need to get out there with the new B&W ND110 attached and try this out!