Riporto una risposta di Roger Cicala (lensrentals) ad un utente dpreview che chiedeva una cosa simile ma su lenti meno specialistiche di un 300 2.8 IS II.
Interessante un passaggio imho, che ho evidenziato:
“ This is news to me about adapters, but we generally only keep lenses for 2 years.
What attracts my attention is that an EF 24-70 f/2.8 was still working after all this time. In a weird twist this is one lens that remained so popular after it was discontinued we kept many (way over 100 copies) for some years after they were discontinued and were working on them constantly -- they are old, they died a lot. Because they were old and had older electronics in them. The ring focusing motors in particular were the limiting factor and we got really good at restacking and repairing them, but eventually they all got too beat up to repair.
So, just an educated guess, but I think the fast horse in this race is "old electronic lenses die".
All that being said, there is a theory that could fit: new cameras have many more focusing steps and quicker focusing calls than old cameras did. I could maybe see that the newer camera (via adapter) could stress the motors and electronics in the lens more than an older camera would . But Occam's Razor says old lenses die is the likely answer.
Purely mechanical lenses do live forever, but not electronic ones.
Roger „
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