To use IS or not.

I have a Canon 400 IS 2.8 lens that I often use with a 2x extender and tripod. The manual says not to use IS when using the tripod. I have read though that it still helps. So far I have not used IS in conjunction with the tripod.

What's the truth?

The manual is wrong ;-) You can use IS all the time with this lens, as Canon itself has stated in many other occasions. The first generation IS created some problems when the lens was mounted on the tripod, but the supertele have second generation IS, that can be kept on even with the tripod.

Juza,

Thanks, I understand that it won't hurt to leave it on. Canon says turn it off to preserve battery life. My real question is does it help to leave it on? Canon says a sensor understands that it is tripod mounted, and in effect disables it, but that one is still using extra battery power. I have read though that it does help get sharper images even when mounted on a tripod.

I should have explained my question better to begin with... sorry.

It helps a lot, even on a tripod - I would always leave it on.

I agree it helps a lot even while on a tripod. Often the ball head is used lose or the gimble head is set for fast tracking so the tripod is merely used to support the weight or there is wind, etc. so the IS will help with vibration. If you are tracking I would use function 2.

Thanks guys. That's what I was hoping.

Juza, I have tremors from severe diabetes. I can still see the IS work on my 500mm f 4L IS lens when the lens is tripod mounted. Yes, the IS works very well when mounted on a tripod.

I have a slightly different take on this Peter, I've had some problems with the speed of the autofocus when the IS is on. I only use the IS when the light is pretty low or when I'm shooting with the 2X converter, or shooting handheld. However with hand held flight shots I don't have it on because again sometimes it seems to slow down the autofocus. I would experiment with it and see what works best for you.