Originally posted by Douglas_of_Sweden I wouldn't follow this idea. You would still be missleading the camera to introduce unvanted blurr as soon as you shoot at a focal length below the setting you have entered. I'd recommend either turning the shake reduction off, or set it to the shortest focal length, unless you know that you are going to work for a while at a specific (and longer) focal length...
No, the camera wouldn't be introducing blur at FLs other than that which you input. The camera will still be moving the sensor to counteract the shake, it's just that the movement will be less than optimal for longer FLs, and more than optimal for shorter FLs.
Iy you want the best SR possible, then you have to enter the FL at which you're operating. If you don't want the hassle, then you should enter as I've described.
You'll notice that, most of the time, with lenses of moderate zoom ratios, you'll be getting about 50% of SR at the extremes of the zoom range. What this actually means is that the camera will be reducing shake by a factor of 2. This means you get a worst case of 1 stop of shake reduction. Which is pretty worthwhile, I reckon.
You get 100% SR when the FL is equal to the value input to the camera (obviously). If everything were perfect, the camera would exactly counteract the shake, so you'd end up with no blur at all - but of course everything isn't perfect, so you get around 2-3 stops worth of shake reduction (usually, depending upon shutter speed).