Why turn off IS?

I just left work early to come home to my 20D that just arrived and it is a peace of pure sexuality. I ordered the 17-85 IS with it as I wasn't about to spend $1200 on the 24-70L that only gives me an effective wide angle of 38mm. Anyway, this is my first lens with IS and it has the option to turn it off. I'm just curious when, if ever, it's best to turn it off.

I don't have any IS lenses, so others may be able to give you a better answer, but two reasons that I've heard for turning it off would be:

1) When mounted to a tripod, it's not good to have it on
2) If your using really high shutter speeds where it wouldn't be necessary, turning it off could allow your camera battery to last longer before charging again.

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Originally Posted by Arctirus

I just left work early to come home to my 20D that just arrived and it is a peace of pure sexuality. I ordered the 17-85 IS with it as I wasn't about to spend $1200 on the 24-70L that only gives me an effective wide angle of 38mm. Anyway, this is my first lens with IS and it has the option to turn it off. I'm just curious when, if ever, it's best to turn it off.

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Biggest reason to turn the IS off when you don't need it (which is most of the time) is that the IS is working every time your finger is actuating the shutter button to take the picture/activate the AF of the lens, even when it's not needed, which in turn, drains the battery faster than using the lens without IS. Rule of thumb is that you turn it on when your shutter speed falls below the 1/focal length rule during hand held shots. It really does work though, even well below the 1/focal length rule.
Here's an example of when IS comes in handy. I shot this wanting the blurred effect of runnng water during broad daylight but didn't have a tripod or neutral density filters, so I shot at ISO 100, stopped down to f32, 1/6 second @135mm on a 28-135 IS lens.

<img src="http://gallery.photo.net/photo/939510-lg.jpg">

Very nice, that's a very steady shot! Thanks for the tips guys!

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Originally Posted by drifter136

I don't have any IS lenses, so others may be able to give you a better answer, but two reasons that I've heard for turning it off would be:

1) When mounted to a tripod, it's not good to have it on
2) If your using really high shutter speeds where it wouldn't be necessary, turning it off could allow your camera battery to last longer before charging again.

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There are different version of IS. One type is found on consumer lenses and should be turned off when on a tripod. The L lenses use an advanced system that can be used on a tripod and are actually used that way to eliminate the need for mirror lockup.

The IS system is only active when the shutter is depressed. I have used IS lenses since late 2000 and have never seen any battery difference with IS on or off, both film and digital cameras. I truly doubt that the tiny bit of intermittent extra juice is even noticable. Mine IS is always on these days.

IS has no effect on fast shutter speeds but doesn't do any harm either. Leaving IS on 100% of the time is not a problem

Great! Thanks for the info Micheal!