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?