I mounted my new Tokina AT-X ProDX 124 on my 20D and it immediately locked up when I turned it on. I had also inserted a recharged battery. All I got was some lens activity but no LCD menu screen and it would not release the shutter. Also the camera would not turn off. I removed the battery, waited,and reinserted, the cameracame back to life and all worked fine. I went by the camera shop where I purchased the Tokina to inquire as to what could have caused the problem. The response was that the older 511's occasionally cause problems with the 20D. They said that a Canon Rep had advised them of occasional problems with the 511's and recommend only using the 511A to avoid the lockups. They said that if the lens had been the problem I would have gotten the dreaded err99.
Any one else experience problems using 511's vs 511A?