Originally posted by yusuf
Any idea what could have caused this?
Mostly just age, though heat can cause the cement to break down faster.
It is repairable, but it is costly to have it done. I've always wanted to try it myself, but never had a lens with the problem that would be worth repairing. I have saved a few good cemented elements out of otherwise junk lenses to practice on if I ever get a good lens that I would actually want to save. The cement isn't cheap, so I want a really good reason to attempt it.