I bought it three years ago and found the images soft, but never tested it until now. With the 5D Mk IV on a tripod, I zoomed out the lens and made sure the range was large enough to have a shallow depth of field so I could look for front or back focus issues (which I expected). With the Spyder LensCal target I could clearly see that most of the time it just didn't focus very well. In Live View mode, which uses a very different focusing technique, image sharpness is almost on par with the Canon EF100 L Macro, which I also compared. This is a famously sharp lens, so the zoom lens zoom is impressive. The glass is clearly fine, but there is something wrong with the mechanism for focusing through the lens. Be sure to check your lens before 3 years is up! Maybe they'll fix it under warranty. Mine is going to a Canon repair shop and I'm afraid to imagine how much it will cost.
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