7D Auto-focus sound?

[dogcatcher]

Over the past year I have been suspicious that something in the 7D auto-focus might be disturbing insects when I photograph them with Canon zoom lenses. Sometimes it has been when the camera gains focus and other times only when I commit to taking a shot. Last week on 10 consecutive occasions a moth flew off as I took the picture, as soon as I switched to manual focus I got a shot. Not all insects are disturbed and I haver never experienced this when taking voucher photos with a macro lens set to AF
I know that AF is meant to work on pixels within the image but it's as if the focusing is emitting ultrasound. Anyone experienced this or got any ideas?

[Sam6644]

I don't think camera bodies make AF noise. I'm no expert though.

[hollis_f]

Originally Posted by dogcatcher

I know that AF is meant to work on pixels within the image but it's as if the focusing is emitting ultrasound. Anyone experienced this or got any ideas?

Does your lens have USM written on it (or HSM if it's a Sigma)? That stands for Ultra (or Hyper) Sonic Motor. So, yes, your lens does emit ultrasound. Now my ears are old, bent and knackered; but my bat-eared friend, who can hear quite a way into the ultrasound range, tells me that the frequencies she can hear from USM lenses are about the same as those used by the local bats (yes, she really can hear them).

So I reckon it's not beyond the realms of possibility for your moths to hear the USM, mistake it for a bat and try to escape.

[dogcatcher]

Originally Posted by hollis_f

Does your lens have USM written on it.

Yes, and some of the more expensive ones even have Ultrasound written on them in red letters. I had assumed that the US motor was shielded but what you say is exactly what happens with some moths, they are off like a bat out of hell. And here I am with a colony of bats living in my roof space - there's gratitude for you. Back to manual focusing or get some clockwork lenses.

[digsmum]

Do birds hear in the same ultrasonic range? I had a prime spot by a feeder with birds on and off it all the time earlier this week. They also seemed to know when I was about to take a shot - and took off, I assumed it was my movement that scared them (even though I tried to be as still as I could) - but maybe the noise from the lens?