Originally posted by sam-joseph A 50-135 might be interesting too.
Have seen one open, (opened it myself), disappointing amount of plastic, the bayonet is metal, but behind that, is a plastic part connected to the rest of the lens with only four tiny screws in another plastic part.
All K-mount pentax lenses after the M series have plastic in them, the newer, the more.plastic.
Some more expensive A lenses have very little plastic in them.
The FA series power zooms, have metal gears for the parts which move the focus and zoom mechanisms, and plastic gears for the encoder wheel that signals the motor movement to the system (new in the FA, not present in the F series).
The lenses are of glass, but again the FA series introduced the aspheric molded plastic on those lenses.
I have seen a FA * 1:2.828-70, with a strange artifact, which was probably caused by a faulty fabricated aspheric molded layer.
If you want solid durable K-mount or screw mount lenses, stay with the M or older series.