Long story, but those lenses use a hyperbolic focus distance thing. I didn't like it, so what I did, is I rolled up the rubber grip on the focus ring, under which three tiny (seriously tiny, disturbingly tiny even) screws appear. It's smeared over with some plasticy goo. You can scrape that off. You'll need an absurdly small screwdriver, something that's normally used to adjust atoms in a molecule under an electron microscope.
Loosed those screws up only just enough so that the ring rotates freely. I forgot before you do this, pick an object 1 meter away from the lens (or 3 feet). Carefully focus until you're sure it's tack sharp. Then loosen that ring, and make 1 meter line up with the marker. Then tighten the screws again. Test out 1 feet, see if it's sharp. Test infinity.
It took me a bunch of times to get it right. What frustrated me was that infinity kept on being slightly off. So what I ended up doing is aligned it to infinity. And not on the infinity marker, but all the way to the very end. I'm not interested in focusing past infinity. Then I discovered that that also maximized how close I can focus as well. It just about lines up on the near distances as well. So, now when I have an object that's about 1 meter away, I can just set it to 1 meter. It's otherwise just about impossible to focus during use. Anything over 2 meters goes to infinity anyway. So most of the time I now just set the focus all the way, and just click away.
Quality wise, the lens is soft at f3.5 but can be helpful in really bad lighting situations. f5.6 it's better. f8 it's very good. f11 is just about its max quality. And it can produce absolutely razor sharp shots. I've taken small crops from it, defished it, and ended up with normal looking photos.
Just like how a telelens is a good "spy lens", the fish eye is also a very good "spy lens". (not that I'm into spying or anything like that). You walk into a room, or museum, say. You snap a couple in a few directions, without knowing what's in the room, really. Then when you get home, you can pan around and zoom in, and discover all kinds of things.
Another fun thing is video. Put the camera on a tripod, high up, in the corner, and aim it so that it records the entire room all in one shot. Focus to infinity. Start video. Walk away. Then go play with the kits in the room. Amazing what you end up capturing. Can never do that with camcorders or other lenses. Also works great for cameras that either don't have auto focus during video or where auto focus sucks. Everything is in focus all the time. Problem solved ;-)
