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I've a Canon 600D with Canon EF 25-105mm F/4 USM L lens. When I half press the shutter I can hear the US motor sound trying to focus on my subject, however I hear another sound, it's lower than the sound that the US motor makes when it's trying to obtain focus. And when I raise my finger of the shutter I can hear that the sound of the US motor has stopped while this other sound is still going few seconds and then it stops. I wonder what is this sound?

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The sound that scares me is the click my 28-135 makes when I insert the batteries to my camera into the battery grip, even when the camera is turned off. Obviously some power has been put into the system ready fro turn on, and the lens knows this. It scares me every time. I worry I'll wake up with the lens at my throat.... –  Dreamager May 8 '12 at 18:44

2 Answers 2

up vote 14 down vote accepted

You're hearing the IS start to activate.

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And yes, it's completely normal, and nothing to worry about. –  Fake Name May 8 '12 at 9:10
    
so why I'm not hearing this sound on low quality lenses (like the kit lens 18-55 EF-S IS II)? –  akram May 8 '12 at 14:05
    
or does it exist but I can't hear it because it's hidden by the loud sound of the auto focusing motor? –  akram May 8 '12 at 14:25
    
Probably two reasons - larger lenses are going to require more 'oomph' for the IS so it may be a bit louder. And as you suggest, it may be hidden by a louder AF system. –  rfusca May 8 '12 at 14:50
    
Indeed, I have confirmed that the 18-55 lens has a silent IS, while the IS on bigger lenses have a click sound and a whirring sound. It's a strange phenomenon. –  Nayuki Minase May 9 '12 at 15:34

Yep - this is the image stabiliser kicking in. Note if you flick the switch on the lens to turn IS off, the noise you describe will cease. (But obviously, you will then lose the benefit of it). In short - it is perfectly normal, nothing to worry about, and helps you achieve sharper shots by minimising the effects of camera shake.

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