Originally posted by Bertrand3000 It's true! Thanks for pointing it out. That said, I do have the Sigma 10-20mm, and I felt that not being able to zoom past 20mm was limiting, and I now use a DA 16-85 which is less wide angle than my Sigma 10-20, but I know I can increase focal length without any lens change for the session. That said, with 16mm I still feel sometimes limited in field of view. I suppose lens change is inevitable anyway. Since OP already has a 17-70, he really needs a real ultra wide angle lens.
Not only that, as to16mm from the DA 16-85mm, for subjects having linear features where linear distortion is of concern (buildings, or parts of buildings in the frame, interiors, street light poles, cityscapes, etc.) the distortion will be quite high, where the Sigma 10-20mm f/3.5 will present very low to nil distortion from about 12mm, and has constant aperture design for good low light capability at all its focal lengths.