Hello,
I have a Tamron 90mm lens. This lens has a screw drive AF mechanism
(I'm talking about Nikon mount). To switch to MF, one can simply slide
the focus ring back, and the AF disengages; it's then possible to focus
manually without setting the camera to MF (which retracts the AF
screw).

The problem is the following: When I slide the MF ring back,
disengagament is not complete. I can focus, but sometimes the part that
mates to the screw also rotates (I can see this when the lens is not on
the camera). Obviously, this way I may damage the camera or lens if I
am not careful. Using AF, there is no problem; using MF is also
perfectly safe as long as I switch to MF from the camera too,
retracting the screw.

So, has anybody else had a similar problem with this (or another,
similar lens), did they get it fixed, and how much did it cost? I don't
really see a reason to fix it as it does not affect operation (except
in slowing me down a bit), but if it's cheap I may send it at some
point.

Thanks.