Sigma 17-50 f/2.8 Negatives, Customers reviews states that this lens is as good or better than Nikon 17-55 but some are having problems with AF & build quality.
Jeff Hollis Review: Feels sturdy, but the internals are
cheap: I bought this lens to use with my 7D for video, and it was working great. Range
of the lens, and constant aperture, plus the stabilization are great!
Unfortunately, my lens got "bumped" during a shoot. It wasn't dropped, someone
bumped me with their camera, with some camera to camera contact. Their lens was
fine (canon 70-200), but this one felt like something came loose inside. It no
longer focused, and was very wobbly near the camera mount.
I'm one who
opens things up to see what's wrong, and I decided to do that in this case, as I
needed the lens the following day, and didn't really have an option to take it
back. Once I found the problem, it wasn't surprising why the lens was wobbly.
There are screw mounts all over the inside of the camera holding the components
together, in my opinion that should be metal, but instead they are flimsy
plastic. I epoxied the strut back on, very well, actually. Reinforced the sides
of it, reassembled the lens, and after a test it was no longer wobbly or odd
looking. The focus also began working again, but not properly. It just cycles
focusing in and out, seldomly finding a focal point. Fortunately, this doesn't
matter a TON for video, as I can't auto-focus with it anyway, but it's annoying
when I'm setting up a shot.
While the lens did perform well, it's not
made to take even a small to moderate bump.