Whether you can see the difference through the view finder depend on the length of the lens. With my 70-300 IS, at 300mm, when you half depress the shutter, it is immediately apparent that the IS has activated. With my 17-85 IS or 24-105 IS nothing appears to happen in the viewfinder but from
this shot and
this shot in my gallery, both at 1/13 sec hand held it clearly is working
