Originally posted by rhodopsin Actually, if focal length is indeed somehow tied to SR operation, such as shutter speed being the deciding factor on where SR should occur, wouldn't choosing the long end of the zoom be most effective?
No. The shutter speed does not affect the SR function. The SR will try to compensate for any perceived shake by moving the sensor by an amount relative to the focal length used (either read from the lens or in the case here , from an input FL)
If the camera thinks/knows you have a long lens on the camera it is aware that a certain "shake" of the camera will lead to a more pronounced blur than with a shorter FL and thus will compensate by moving the sensor to a greater degree.
Thus if you input 200mm, and use the zoom at 80mm you will get an overcompensation from the SR. This is not good.
If you input 80mm and use the zoom at 200mm you will not get enough compensation, but this puts you in no worse a position than not using it at all.