I'd also recommend the Canon EF-S 10-22mm. It would be one of my top choices for great image quality, unusually good flare resistance for an ultrawide lens, plus excellent autofocus performance.
Personally my close second choice would be the Tokina 12-24mm f4. It's an excellent lens, too. Very close to the Canon in most respects and possibly even better built (it's "L-like").
I've been using the Toki 12-24 for a number of years and have just recently bought a copy of the Canon 10-22 and am starting to use it too.
There are also:
Tokina 11-16mm f2.8. This is another very nice lens,might even be sharper than the 12-24, but also a bit more expensive and more prone to flare. more expensive than the 12-24 and more prone to flare. Also, to get f2.8 - which most people really don't need on an ultrawide - the trade off is a very narrow range of focal lengths.
Sigma 10-20mm f4-5.6 is one of the lower cost options. Sigma 10-20mm f3.5 is a more expensive and larger lens. It also uses larger than usual filters, more expensive 82mm. Most of the other lenses listed here use 77mm filters.
Sigma 8-16mm f4.5-5.6 is the widest lens available for APS-C crop sensor cameras (short of going to a fisheye). It cannot be uses with standard filters, due to a strongly convex front element.
Tamron 10-24mm f3.5-4.5 has the widest range of focal lengths in a single lens and is one of the more affordable choices. A lot of users report it's a bit soft at the 24mm end of the zoom.
If you prefer a prime instead of a zoom, a Rokinon 14mm f2.8 is an affodable option. Note that this is a manual focus and manual aperture only lens, so rather slow to work with. But it's got decent image quality. It also sells branded as a Samyang, ProOptic, Bower, Vivitar (13mm) and some others. All are the same lens, so shop around if you're interested in this lens. Note that it's another lens that cannot be used with standard filters, because it has a strongly protruding front element. All other 14mm and 15mm prime lenses are extremely expensive.
Yes, the Sigma 12-24mm f4.5-5.6 would work, too.... However I wouldn't recommend it because it's actually a full frame capable lens and quite a bit more expensive than most of the above. Sort of a waste, you would be paying extra for full frame coverage and not using it at all on a crop camera such as your 70D.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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