1. A problem often suffered by push-pull zoom lenses. Such lenses have their focal length
settings held in place by friction, and sometimes this isn’t enough to counteract the weight of
the lens.
If you have a lens which slowly slides, changing focal length, when it’s pointing up or down
then you have a lens which exhibits zoom creep. Two-touch zoom lenses usually do not suffer
from this problem to the same extent. Some lenses have locking friction rings to help deal
with the problem.
taken from http://photonotes.org/cgi-bin/entry.pl?id=Zoomcreep
