I just examined my 28-70/2.8 Sigma very closely. The zoom and focusing
rings are plastic, but the only way I can tell is by touch -- they don't
feel "cold."
All the other external parts are metal. Not being willing to dismantle
the lens, I don't how many other parts are plastic. Anyone out there
have a difinitive answer?
By the way... I have no objection to plastic lenses (other than the
mount), IF the "right" plastics are used. For example, the gears in your
car's window crank have long used a very tough form of nylon called
Delrin. It's inherently slippery (which translates to low friction and
low wear) and you can bang on it with a hammer. But I don't think
anyone's made lenses with such plastics.
I will repeat (for the umpteenth time) that Zuiko lenses are also
"guilty" of poor design -- the helicoids are all-aluminum (rather than
brass and aluminum).
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