Ken Rockwell has done some tests comparing five wide zoom lenses designed only for digital camera with APC-size sensor. Here’s what he says about each lens
“Nikon 12-24mm: The Nikon has the best optics, the best focusing and the best everything.
Tokina 12-24mm: It has the fastest focus, the fastest aperture and has the best handling of all third party lenses with no weak points.
Sigma 10-20mm: The Sigma is also the least sharp. You won’t see this unless you look hard. If you do then get the Nikon. If not, the Sigma can make spectacular images, especially at 10 mm.
Tamron 18-11mm: After using it I realize that it has optics as good as any other third party lens, not that you’ll be likely to notice for normal 12 x 18″ (30 x 45 cm) prints.”
Canon 10-22mm: The Canon 10 – 22 mm is better than any of these four lenses, including the Nikon.What makes the Canon so superior is its vanishing low distortion.