I am going to view a used Canon 100-400mm later today and I know the zoom works using a "push-pull" method. I haven't tried one of these out before but just checking on the following: When I try it out should the "push-pull" be a very "smooth" process. Or do some of these develop a rough feel to them. In other words should the sliding back and forth of the lens have a very smooth feel to it. Thx.