Originally posted by mikesbike It seems I read a recent post of a newer plastic Pentax lens did have some part of it come off. One of the less robust plastic lenses of the Pentax line is the DA 50-200mm, even the WR version. Even if working properly at first, I have known of some such lenses that have developed wobble in the zoom components, and zoom creep when pointed downward.
Just for examples, if you were to handle the the DA 18-135mm and then the DA 18-55mm and DA 50-200mm, taking some shots with each and using their zoom mechanisms also, you would definitely notice a difference in the superior quality of construction as well as the superior In-use performance of the DA 18-135mm lens. And yet all are of plastic construction.
Plastic materials should not be demonised, but I see that many posts concentrate on personal feelings. As important as it might be, "user experience" is not everything.
There are more objective considerations. Personally I'm crazy for vintage lenses, but I reckon that the judgement about modern lenses should me mainly based on other things.
I think that the use of plastic materials is due to two different reasons.
One is economical. Most vintage lenses would be expensive to manufacture in present days. Often the all-plastic built is a sign of cheap construction. This is especially true if we consider some consumer zooms, like the silver FA I mentioned.
Another is practical. In-body AF motors can't be stressed by a heavy construction. Even modern expensive zooms can't use powerful in-lens motors because they would affect battery life and probably also physical dimensions (high speed zooms are already huge!).
The number of glass elements has grown a lot from the old days, and f/2.8 zooms have large glasses. All that optical glass weights a lot, a weight that is sustained by the sleeves where the cams engage. These components should be thick and durable, but the weight the AF motor has to move is already considerable. So the total weight has be contained.
Prosumer zooms are usually rather well made. The only problem is their resistance to minor front knocks that would have been inoffensive not so long ago.
Cheap zooms often have another problem. After prolonged use there is some play, mostly affecting the focusing sleeve.
It comes as no surprise, given the way they are made.
Those who have opened a plastic zooms know what I'm talking about. I would be surprised if the play didn't affect optical performance.
Repairability is not a secondary issue. I'm not at ease with programmed obsolescence, especially if I've spent a lot of money on a high-performance lens. As usual, your mileage may vary...