If one is looking for a high quality ultra-wide lens to use on a full frame I would be inclined towards a prime 14mm, either Tamron @ $1100 or the Canon @ $2000 (rough prices). Asking to zoom at these focal lengths might be pushing the technology limits somewhat a bit too much? I own the Tamron 14/2.8 and am very pleased with it. On Milky Way star photos (on film), an extremely tough test for any lens, the Tamron 14 does show some coma and vignetting in the corners of the frame, at f2.8. But it is still tolerable. Since the exposure is already 20 minutes long on Fuji 800 film, any lens that would require closing down is IMHO rather useless for this type of photography. For normal photography the Tamron does not show much shortfalls, but I suspect that if one really looks for it, it should be possible to find Chromatic Aberration in the corners. At $2000 the Canon 14mm should be better, perhaps. Since the Tamron 14 has ended up being my favorite lens for landscapes (both on film and on a D30) if I had to do it again I would simply pay the extra $1000 and go Canon, simply because it "should" be as good a lens as current technology can make. By the way, for reference, on chart testing, I found my Tamron 14mm to be sharper than my Canon 28/2.8, right across a D30 frame (too much bother to test on film). To see how much coma you get in corners on the Tamron 14mm:
http://www.geocities.com/samirkharusi/largemw.htmlAgain, for comparison, regarding how tough a test star photos can be, I have never been able to get similarly sharp star photos with my other Canon primes set wide open (Canon 28/2.8, 50/1.8, 100/2.8 macro). The stars end up being larger blobs. I did say that starphotos are a very tough test...