Why turn off IS

[rsp]

Another reason has to do with certain lens IS system failing while
mounted on a tripod. IS looks for movement, and if none is found the
mechanism can sometimes go into shock and start to "correct" what is
not there.

You should turn off IS for long-duration tripod shots, but your
reasoning is slightly off. The IS isn't going into shock or anything
like that. Rather, IS is always unstable over long periods of time -
the mechanisms are designed to correct movement over short periods of
time, but not over long periods of time. If you're taking a 10
second exposure at night the IS mechanism is going to drift and cause
blur - the mechanism is not capable of detecting this drift and
correcting for it, as it is too slow. For those physics/engineering
types out there - it has a very short time constant.

I have experienced that...

Most likely IS drift would add blur whether you're hand-holding or
not. However, for hand-held shots the added blur is inconsequential
compared to the amount of camera shake your hands will introduce.
Once you have a steady platform the IS system itself generates more
blur than the ground does - and so it should be turned off.

I don't think IS adds any shutter delay when it is not needed - it
just won't activate unless you hold down the shutter release halfway
for a second or two. It probably does drain the battery.

I usually just leave it turned on except when taking long-duration
tripod shots. Compared to my old P&S, my XSi is battery paradise -
so I tend not to notice the drain so much.

--
rsp

 

[Bohdan]

it is so miniscule that, at the end of the day, it makes no difference.

It's definitely not "miniscule", but the number of shots that can be
taken by one modern battery do make it effectively a non-issue under
most normal circumstances. The only times battery life and use of IS
have ever been an issue for me are when I've found myself without any
ability to recharge for a several days or I've been shooting
continually in very cold conditions.

To put some figures to it, shooting fast action sports using IS
almost continually on multiple lenses from the 70-200mm f/2.8 through
to the 500mm f/4, I can usually go a full day on a single battery and
only rarely need to switch to my backup. By comparison, shooting
closer up with non-IS lenses like the 24-70mm f/2.8, I can easily go
a full week before I need to start worrying about where I've left
the spare. That's quite a difference!

Andy

How many shots, in both cases ?

--
I may be, and probably am, completely wrong.