My first L-grade lens and wide angle zoom. This lens in very soft in the corners when used on a full frame camera body, even when stepped down. Center sharpness is excellent, best IQ is between f/5.6 to f/8. I don't use this lens on my full frame camera anymore, being replaced by an EF16-35F4L may have something to do with that. But the 17-40 works very well on my cropped sensor camera bodies. This lens sees a lot of use on my 70D. No IS is really the only drawback when used as a walk around lens.