Hector Javkin , Feb 11, 2010; 05:03 p.m.
I assume you're thinking of a protective filter. On this lens, the hood offers so little protection, because it has to be shallow for the wide angle, that a protective filter seems desirable. However, any filter you put on is going to put a piece of glass further out from that same shallow hood, so that it is more vulnerable to stray light. I wound up not putting any filter. If you insist, get the thinnest one that's most resistant to flare. Bob Atkins, both somewhere on photo.net and on his own web site, reports on his tests of the resistance of different filters to flare. Follow his findings. Don't have time to find them for you, but it's not hard.