Not to thread crap but I owned one of these at one point and got rid of it quickly. Chromatic aberration is absolutely horrendous. With the 7d and an 8mm fisheye, the results are much better. Plus the lens wont cost you a fortune should some one rocket a board into it. Just throwing that out there.
The moded century mk2 lens for the dvx will have the same thread size as the hpx170. I'd look into one of them as well as there is a lot of them going around on this and the skateperception forum.