Reflective patches on interior lens element

Question

Kinda what it says on the tin. I picked up an old generic lens for a few € at a flea market, and there are what look like silvery, reflective "splats" on one of the interior elements.

It looks like this, except there's a few of them:

Source: Markus Keinath's... is it a blog?

I know it's a kind of separation (confirmed by the linked blog and a bit of screwdriver-turning), but my question is this: what causes this kind of separation, as opposed to the more "classical" effect of chromatic stripes? It seems rare - I had a hard time finding anything matching it.

Answer 1

Camera lenses are complex arrays of multiple lens elements. It takes several lens elements with different figures (shapes) plus some are made of dense glass others are less dense. The idea is to mitigate the 7 major lens aberrations that do mischief.

Now some elements are air-spaced and some are cemented together. For many years, the cement used, because of its clarity and index of refection, was Canadian Balsam, a refined pine tree resin. Because this cement is organic, it can become a mouthwatering tidbit for several species of mold and mildew. Sorry to report that your only remedy is to disassemble, clean, re-cement and reassemble. Today, lens makers use a synthetic resin not so susceptible to mold.