I've just bought a Canon EF 400mm f/2.8 USM IS, and although I have perfect focus, the USM still works. It also works when I press the shutter button halfway. But I set the focus to another button. Is this normal? Are there any configuration settings that I am missing?

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    Are you sure it's the focus motor and not the IS? – Philip Kendall Aug 4 '15 at 18:45
  • What camera body are you using? – inkista Aug 4 '15 at 20:01
  • Which AF mode are you using ? Which Canon Body ? – Olivier Aug 4 '15 at 20:52
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    That sounds liek continuous focus, like you changed the focusing mode, or switched to the sport image program. – Guffa Aug 5 '15 at 0:24

What you're noticing is not the focus motor but the image stabilization system. If the focus motor were moving, the focus would change. The IS system will start up when you half-press the shutter button.

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