Having used Minolta film cameras and their wonderful 24-105 I had hoped for similar performance from this beautifully made and fast focusing zoom from Canon.
Unfortunately it’s performance doesn’t match it’s looks. Tested with a Canon EOS 20D - performance at 24mm was poor at the edges. Considering this is really a film camera lens and the smaller 20D sensor is only using the center of the field I was surprised at the amount of fall off. At 28-85 performance was better but nothing special. Strangely enough test images with Canon EOS 5D were better than with the 20D but still dropped a fair bit at the edges.
Overall I’d have to say that, although it’s lovely to use, performance from this lens has been disappointing. Maybe I’m being a bit over critical here. Actual results at reasonable apertures are fine - just not as good as I had hoped they’d be. For now I think Canon’s 24-70 2.8L or Canon 24-105 f4 L IS are much better options - but at a price.