I used a 400mm f/5.6 lens with my Canon DSLR once back in the day, and I didn't have the problem you're having. I let Ekos autofocus with the lens, and it worked. The camera did not try to autofocus before each shot.

The only thing I can think of, is my cameras are always setup with back button autofocus (the shutter button doesn't autofocus). On the 450D, change custom fn. 10 (shutter/AE lock button) to 1 (AE lock/AF). That will cause autofocus to be moved from the shutter button to the * (asterisk) button. Maybe that will help.

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