Actually that is because threadlock was applied to them in the factory. The
screws themselves are rather short surprisingly (2.5mm only, even for
FA*300/4.5), and 3mm for some heavy lenses.
Alan Chan
http://www.pbase.com/wlachan
Depending on the location of the screw, one can often pop it loose by
putting a screwdriver into the head and gently tapping the back of the
screw.
Obviously, care must be taken with this, but I have found that lens mount
screws, especially, are generally rather tight, and have lots of support
behind them.
