(Not to be confused with fog filter.)

After some filter-factor tests made yesterday, I went to check more closely on my green (11), which appeared to be lowering contrast in an unexpected manner. It needed cleaning, as did several others, so out came the Eclipse and a PEC wipe. Two of the others cleaned up fine; the green and made polarizer not as well. The outer surfaces appear fine, but there is some sort of fogginess which must be between the glass layers. I haven't encountered this before.

I keep my filters together in a stack using metal end caps (Stackcaps or some such trade name).

Is this just likely an aging issue? I would guess these are 20 years old.

Thanks. It's always great to have this board for answers!