I recently got a Canon 20D from eBay, and bought new a lovely 50mm 1.8. I also acquired/borrowed two EF fitting Sigma lenses which date back to film EOS cameras:Sigma Zoom - 28-200mm 1:3.5-5.6 DL Hyperzoom Macro and Sigma DL Zoom 75-300mm 1:4-5.6. Unfortunately, these virtually never work on my camera, giving "Err: 99". After a lot of googling and reading, I've read this is typical for Sigma lenses of this age on newer Canon bodies. The reason is that Canon update their protocols with full access to technical material (so their older lenses would be fine), Sigma reverse engineer to fit. This means Sigma lenses may not always work on future Canon bodies. It was, apparently, possible to get a lens rechipped to work with the newer Canons. I need to contact Sigma to see if this is still possible, but some posts I've seen online indicated that they've run out of the replacement chips. Another alternative might be to put a component (such as a cheap extension tube) in between the camera and the lens which prevents communication completely between the camera and lens. Obviously, this would stop the lens from operating at infinity and disable the autofocus, so it's far from ideal. Anyone else dealt with this issue?