Let’s first answer your questions from a DSLR point of view.
1) The IS is in the lens. The auto focusing system is in the camera.
2) If you switch to MF, then the IS should also be disabled.
3) If you are using a tripod, it is recommended that you turn off IS when you are using the autofocus system.
First, thank you for your reply. That was nice of you. Yes, I am using a DSLR .No in-body IS - lens only.
Secondly, and I am not being argumentative, but if your second answer is true, why would I need to turn off the IS in my lens, if it has been disabled when I switch to MF?
My understanding is that:
1) For a DSLR, the IS is within the lens. The camera and the lens work together for autofocus. The camera decides what number of autofocus points to use and whether it will track on single or servo mode and the lens activates the autofocus mechanism accordingly. The lens disables the autofocus mechanism (and thus autofocus) with the MF switch on the lens barrel where this is available, and there is not servo mode, but the number of AF points remains constant .
2) I don't believe IS is impacted by turning off autofocus - I just tried this on my EOS 5DIV and the EF 70-200 f/4 MkII lens. I set the camera with AF and IS, then took off AF but left on IS and the IS worked; finally I turned off IS and there was camera shake. I repeated the experiment on the Canon EOS 80D with a Sigma 150-600c lens @ 600mm, and the results were the same.
3) If you are using a tripod, it is recommended that you turn off IS when you are using the autofocus system for most EF and all EF-S lenses, but some L series lenses are able to sense this and will automatically adjust for the tripod.
I think I understand a little better.
The lens IS is a gyroscope and operates whether your camera is autofocusing or not.
I read my manual and it says IS is working all the time to correct camera shake, not focus.
It recommends turning off IS when mounted on a tripod only to preserve battery life.
My basic premise was wrong.
Well done for reading the manual, so few people do!
IS is actually achieved by a one of the lens elements towards the rear of the lens vibrating at a high rate, so you COULD perhaps see it as a gyroscopic action. IS and focus are discrete as you and I say. IS is recommended to be turned off for more than to preserve battery life. You never mentioned the gear you were using but for many cheaper lenses and even some older L series, the vibration continues and will actually cause image shake. Thus the precept to turn it off for use on a tripod. Some L lenses sense that the camera is on a tripod and turn off IS automatically. Usually those lenses come with an actual manual so you can see the instructions.
There is a new generation of lenses (e.g. by Tamron) that don't have an IS switch because they sense the tripod mount and shut off automatically.
Trevor is correct. AF, MF and IS are independent.
IS helps compensate for the motion blur that could occur from using a slow shutter speed.
When using long telephoto lenses (i.e. large magnification) IS can be helpful in stabilizing the image seen in the viewfinder even if using a high shutter speed where motion blur wouldn’t be a concern.
The instruction manual for the lens will state if IS should be turned off when on a tripod.
"2) If you switch to MF, then the IS should also be disabled."
I think he meant you "should" switch IS off. But it is not automatically switched off when selecting IS and MF. All lenses I have or have worked with have two independent switches for IS and MF. Auto focus is in the lens not the camera. The camera simply tells the lens when it is in focus using the selected AF points. That's why certain cameras can focus in varying conditions with the very same lens.
I know some say to switch IS off when using a tripod but I have never seen any difference either way. So if it floats your boat do it whichever way you want.