This has come up before here, but I'll mention it again. I have a 20 mm AFD lens which had a fogging problem in the glued element. Apparently for at least a short run Nikon used some unusual cement here, and it's not easy to fix. My local repairman did some research and determined that regular balsam was a not used. I tried getting a new element from China using the Nikon part number, but the wrong part came, and I despaired of being able to get the right one from their nearly indecipherable site, if it even existed.
The repairman has done this job before, and prefers not to, owing to the difficulty of alignment, but he tried it this time, with a more modern synthetic cement that was said to be like that used by Nikon. Unfortunately, the lens came out unacceptably soft. He suggested that the cement they used probably has a unique refractive index. He was up front about the possibility of getting the alignment bad, and the resulting lens's softness was very even, so I don't think the problem was that. The attempt ended there, as he was unwilling to charge for the failure, but understandably reluctant to keep trying.
Another poster here later noted that this lens, along with another (the 180 D I think) were known to have this problem, and he had had one repaired by Nikon, but my recollection here is that it was pretty expensive, and not in the US (South Africa, maybe).
So let's hope that the problem is fungus or something else not inside a glued element, of which I believe that lens has more than one.
