i just got my lens yesterday. so far so good, everything looks good, but i have a question about the lens. right where the rear lens cap screws on, i can feel a thin piece of rubber all the way around the edge of the lens, where it mounts to the camera. is this for weather sealing? my 70-200 didn't have that, not that i remember, i sold it, and i don't see it on my 100-400. thanks.