Originally posted by DonV I am a little confused by the "de-fishing" comments; if one is going to "de-fish" in post, then why use a fish-eye lens to begin with ?
As you say- back to filters; as said by others- the CP and ND are problematic, but a graduated filter- just needs to be "large"- I guess!
A wide rectilinear lens will have an apparent distortion at the edges, as the same angle moves thru a wider distance, and thus circles for example become oval shaped. A FE after a distortion adjustment, can actually remove this distortion (anyway Fish eye hemi is designed to do this), and thus look much more natural. And a FE after such adjustment still is likely to have a wider field of view (as the FE center horizontal is not narrowed at all, just the "wings").
I basically always use a FE rather than an extreme wide angle, although I find a FF FE on an APS-C crop sensor (and Fisheye Hemi) gives me the best results. In fact my Pentax 17mm f/2.8 wide open does quite well (on my K-5's).
I actually like them so much I have three FE lenses in Pentax K, one in Nikon F, and one Pentax Q--and the FE look is something I almost never go for.