Canon have always put two main reasons for putting IS in the lens.
First, the IS is custom made for the specific focal length and lens characteristics.
Second, for optical viewfinders lens based IS steadies the view through the viewfinder, this is not the case with IBIS systems with optical viewfinders, it is true for EVF's though.
Thanks for clarifying. I had forgotton about the second point.
So the IS in the lens is designed to compensate for movement in ways that a sensor just couldn't do? I imagine a sensor can only move in certain ways - for example up down left right and maybe tilt? And maybe not all at the same time right? That would be some clever sensor acrobatics if it could!
The in lens is basically a series of small optical elements that move in response to the way light enters the barrel, right? Meaning there is more than just one thing moving. So is in lens IS more acurate too? And what about Hybrid IS? Could a sensor mimic that too or not?