Summary: Wide aperature at both ends. Really sharp images esp center. Some vignetting all the way open. Overall very good image quality but AUTOFOCUS on my copy works only about HALF THE TIME. It often refuses to focus.
Summary: I needed a good deal have a lens zoom 2.8 & this lens she seemed have an excellent quality with a little price but was only one big deception. I used this lens with the Canon 40D, she showed have an excellent and sharp wide but she showed have front focus among 40 and 50mm and that detracts from my work. I tried to insist but the B&H sent another and was with the even blemish.
Pros: Easily Interchangeable, Lightweight, Strong Construction
Cons: Blurry Focus, Difficult to Clean, Tough To Attach
Summary: I really wanted to like this lens, and was excited to receive it after reading many positive, and a few negative reviews. However, after having it for about 5 days, and testing it extensively compared to the 18-55IS kit lens, I find it unsatisfactory. I tested it indoors and outdoors, including using printed out resolution test patterns taped to a mirrored door. Rebel XSI on a tripod, RAW, Live View used to focus.
Summary: I'm on my 2nd copy of this lens. The first went back within 5 days due to the fact it was contantly loosing contact with the camera body. All I could do to fix it was take off the lens that put it back on again and the camera body could see it. My 2nd lens did a similar thing and I thought maybe it just needs to be broken in a bit more. Well no such luck.
Summary: I used this for about a week and half, it was later returned for my money back. Optically this lens it top notch with good build quality. The auto focus seems good in adequate lighting, but it is SLOOOOOOWWWW in low light (when you press the shutter half way down to focus, it starts off fast to focus, but when it get close it does micro-steps into focus. You will notice this more as you have lower light.) How slow you ask?