Photography - Sigma 17-50mmOS
The use of the short primes (35mm f/1.8 and 50mm f/1.8) really opened me up to available light photography, but I'm constantly switching back-and-forth, or trying to zoom with my feet. I decided that it was time to pick up a fast zoom. I tried a couple of options, and was very pleasantly surprised by the Sigma 17-50mm OS.
This lens is sharp. The center of all images is quite sharp throughout the zoom range, even wide open. The borders are reasonable good at f/2.8, but are noticeably less sharp than the center, when under 30mm. Stopping down to f/4, the image is virtually even, across the frame. firstname.lastname@example.org sharpness is comparable or better than the 50f/1.8, at the same aperture.
The Sigma is as fast focusing as any lens I've used, and other than a couple of misses at 17mm-1/15"-f/2.8-ISO800+ focus was bang on. Honestly, this could just as easily have been as a result of user errors... I also tested the Tamron 17-50VC and, overall, I'd say speed was equal, but the Sigma was far less prone to mis-focusing.
There is some minor distortion, at 17mm, but it is pretty minimal, from my preliminary tests. No where near as noticeable as the Nikon 18-105VR. Otherwise, the optics of this lens are near superb. There are some reports of field curvature, which I haven't noticed, yet. I will continue to test and will update this page, as required.
Weight is nicely balanced on the D90; the Sigma 17-50OS is about the same size as the Nikon 18-105VR, but is noticeable heavier. I wouldn't have any problems carrying the body + lens around my neck, all day long. The 17-50 range translates to 25-75 on a DX body; this is the classic "event" range, and I can see why. Strictly shooting indoors, there would be almost no need for a second lens; the range is great.
The OS (optical stabilization - same as Nikon's vibration reduction) works well on this lens. I haven't put it through the rigors of heavy testing yet (only be a few days), but even with my shaky hands, I can easily handhold 1/15" shots of static subjects. No way could I do that without the aid of stabilization.
I wish the Sigma had some form of weather seal, or a rubber gasket, rather than the bare metal visible when mounted; I gather this is due to the larger Canon mount this lens also accommodates. This is my first non-Nikon lens, so the zoom ring spins the wrong way, for me, and the focus ring spins while AF'ing, so I need to get used to these quirks, but these (relatively) minor nits weren't enough to scare me away from an otherwise very good lens.
Here are a few samples I have taken with this lens: