unimpressed with sigma 17-50 over canon 18-55, how should I properly compare?

[theregoesjb]

I have a canon T2i, 18-55 kit lens, and 50 1.8 prime.

I finally picked up a sigma 17-50 f/2.8 last fall. I think its a quality lens but I'm not feeling enthralled with its performance over my canon kit lens. The autofocus and wide aperture are obviously big improvements, but the image quality and sharpness seem maybe slightly better.

I'm not sure if my expectations are just too high, but so many reviews rave about its sharpness and image quality.

I'm comparing RAW file images I've taken with the kit lens (18-55), the sigma 17-50, and my canon 50, 1.8. All at 50mm (or 55 for the kit), f/5.6, and on a tripod. The 50mm 1.8 is the clear winner, while the sigma and kit lens are roughly neck and neck for sharpness.

Should I be seeing an obvious difference between the kit and sigma?

 

[jaomul]

The canon 18-55 kit is quite good. This faster sigma will probably only be noticeable better in low light when you need f2.8. it should also give you less depth of field when needed

 

[weepete]

Primes are usually sharper than zoom lenses, though I think some of canon's newer L series zoom lenses are sharper than some older primes.

Sigma 17-50mm f/2.8 EX DC OS HSM Lens Image Quality

Suggests you should be seeing a significant difference at the 50/55mm end of the range at f5.6 though.