Originally posted by From1980 I`ve heard this too, but I wondered what the actual incidence of any problems is. Anybody had real damage to a lens needing repair from not disengaging AF before touching the focus ring? I forgot to a number of times with a non-quick shift lens that seems not to mind. Is it just a small chance of strain to the motor and maybe manufacturer`s are being very cautious?
I don't know about lenses, but I've seen a lot of things that have a "mechanical fuse", i.e. something inexpensive/easy to repair that breaks before damage may occur to more critical parts of a system. It would certainly make sense to design an AF lens like this, and also for the manufacturer to urge caution when operating the lens in MF because of this very reason.
My Sigma 30/1.4 (screwdrive/non-HSM) has a focus ring that is always engaged with the AF motor. At some point, the focus mechanism locked up, evidently a mechanical fault. My memory is foggy as to whether this happened before or after someone at a party tried to zoom the lens with the focus ring, though it didn't break immediately after the mishandling.