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For lens with wide zoom range, it's not unusual that lens creep will happen sometime in its life. This phenomenon occurs as zoom ring loses friction and cannot hold the weight of the large front element. Canon EF 28-135mm f/3.5-5.6 IS USM is one of those lenses that have this problem.
Step 1Material: elastic hair band
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All you need is an elastic hair band. The type required for this fix is the
flat one as regular rubber band is too thick. The one I use is
Goody black elastics. For aesthetic, I'd suggest using black one as it blends in with the lens.
thanks for the great write up and fix!!! this creep was pissing me off.
As for the knot question, while (like tkcom said) the band itself is too small to tie a knot, you could tie something around the band – whether it be a small piece of string or another band.