Well, that's the thing. New set of three Kenko AF-S compatible rings (36mm, 20mm and 12 mm) retails around 150€, I could get it with around 110, but it's still a lot of money I'd probably have more use for other more important things.
Reversing rings seems to be about 35 €, but I think it could be a bit off a hassle.
Fun lens I can tell you that. Probably the best ultrawide for crop sensor system. Focused quite close at wide end if I recall correctly. I miss that since my current FF ultrawide Nikon 20-35/2.8 can't focus under 30cm. And the FF Nikon 14-24/2.8 costs a motherload. :faint:
I really wanted to get the Pentax/Tokina 12-24mm f/4 constant, but I couldn't really justify the $200 price difference, especially when the Sigma is (reportedly) so good optically for its price, has 2mm extra on the wide end, and the 12-24mm isn't weather-sealed either.
If I end up really disliking it, I can sell and get the 12-24, but everyone I know who has this lens likes it. I'm not worried.
i'm just wondering, what is the point of all these lenses, by which i mean what makes this one different from all the others posted here? is there a site with all the information (basic to complicated) that i need to find all this out, what exposure, "f/2.8 APO" and so on are.
Well, I guess wikipedia would be the best place to look for broad coverage of what goes into a lens.
The point of all these lenses is to cover all possible shooting situations and get the best possible shots for those conditions. A large aperture zoom like the one above is ideal for indoor events like weddings and so on, and also for sports, portraiture, and more. The 10-20mm I mentioned on the last page is an extreme wide-angle lens (but rectilinear, not fisheye), and it's better suited to landscapes, architecture, and shots where you want a bit of weird/playful distortion.
For the lens above, 70-200mm is the focal length--in this case, a zoom with short to long telephoto. f/2.8 is the max (and, in this case, constant throughout the zoom range) aperture. APO stands for apochromatic, which means that it has better optical correction for chromatic aberration (aka purple fringing). HSM is short for "hypersonic motor," which basically means that it has an in-lens motor that drives the autofocus, and does it silently (similar to Canon's USM). Macro means it can focus at a shorter distance from the lens than most lenses--in this case, it's only a 1:3.5 magnification. The best macro lenses go to 1:1 magnification, but these are typically dedicated prime (fixed focal length) macro lenses, not zooms.
I'm using it with kenko CPL - great combination 
