I hope someone who owns the RF 100-400 can help me out. Apparently, I'm having no luck with this lens (or maybe I am?). I bought the lens from Canon a long time ago and the AF didn't work. I have since acquired a new copy. The AF works. The lens barrel locks properly when extended and doesn't wobble. However, when I look inside the front of the lens, I see the lens group moving when I, for example, gently move the lens from left to right and back again. So, it seems to be loose on the inside. I didn't notice that with the other RF100-400. I took some short test photos indoors. (It is raining outside) AF, IS, and quality seem fine. I haven't done a PixelPeep test yet to determine if alignment plays a role. When you have the lens, do you also see the entire assembly moving inside when you move the lens?