So I bought a new sigma 18 - 250 lens, and I noticed that when I am looking downwards to take a photo (at an angle of 45 degrees downward or more) the part of the lens that moves to zoom slides out from the spot that I had it at. However, this does not happen if I have the lens fully retracted. The same happens when I look up and the lens is zoomed out, except that it slides in (although this does not happen if it is completely zoomed out). Is this normal or do I have a bad lens? If this is normal, is there a way to prevent this from happening? Thanks for the help!