I don't own a zoom lenses but I would guess that if you pointed one straight up and slid the zoom ring up, that over time it could slip down changing the fl of the lense. I imagine that an older lense would work better as it would have become loose over time.
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Tak FC-50
Tak FCT-65
Tak FC-100
"SCUZZO" 300/1.1
Camera Nikon F2 / Mamiya press 6x7 ,6x9 yay Film
Lenses Yes Many - lol
Mount - GoTo by hand and eye