When I change lenses I normally go into the IS menu and change the associated FL to match the lens I've just put on. I noticed in the FL menu options there are gaps (i.e. there's no 17 only a 16 then an 18). If I'm using my 17.5mm .95, should I set the IS FL to 16 or 18? Does it make a difference? Is it better to select a shorter FL or a longer one if the specific FL for the a given lens isn't an option? What about when using zooms (which I don't use that often)? If I have the 12-50 on and zoom in to say 35 but have the IS FL set to 12, will that make any difference?