I used the Tamron 19-35 f3.5-4.5 and the Olympus Wide Angle Extension WCON-08B (.8 wide angle) which on my Canon 10d/ 1.6 crop factor - gives about 24-45mm - I find no degradation in sharpness and very little CA, although flare can be a problem with no hood. I did only mild USM and perspective correction in Photoshop.