There are several choices in the 17-50mm range for crop sensor cameras, and rather than doing separate takes for each, I thought a bit of comparison would be better. The summary, though, is they are all pretty good. Whichever you choose will probably leave you satisfied. An important generalization first: all of the 17-50 lenses are sharper at the wide end than at the long end. If you think you’ll be shooting mostly at 50mm or so, you might want to consider a 24-70 or similar range lens or similar range lens.

Nikon 17-55 f/2.8: The most expensive of the bunch by a good margin. It doesn’t have vibration reduction. It autofocuses more more quickly than the Tamron, which has a micro-motor rather than USM motor. It has less distortion (although it’s still significant at 2%. It’s built like a truck and would be my choice for war-zone photography. Otherwise, it’s really expensive compared to the others.

Sigma 17-50 HSM OS: is as sharp as the Nikon in the center, and at f/4 is as sharp in the corners too. Maybe even sharper than the Nikon at f/4. It has more barrel distortion at the wide end at 2.9%. That is easy to correct in photoshop, but correcting distortion also decreases resolution, particularly in the corners. Probably as good as the Nikon except for autofocus in low light.

Tamron 17-50 f/2.8 VC this lens has excellent vibration control, as does the Sigma and Nikon. It’s not quite as sharp as either, although it’s in the same ballpark, and it is less expensive. It has even more barrel distortion than the Sigma, though, at 3.3% at the wide end. Has had some autofocus problems with some Nikon cameras where only the center AF point is active.