[robski]

I suspect it is more to do with the way the servos work. Basically a servo needs to see an error so that it can make a correction. Some servos designs are based on the fact it needs to induce a small error itself by moving a part to findout what is happening. If you like to find which way is up, right, left or down. I have no idea how the Canon IS servo works other than they seam to have a gyro motor. My experience is of the servos used to track the laser beam on CDROM and DVD. In these you have a focus and tracking servo to maintain to laser head in the correct position.

So from what I have heard from users of this lens if you have IS on using a tripod the the image is not sharp.