During a nice nature trip, sitting at a table with a bottle of wine, somehow we had the ridiculous idea to test a lens as burning glass (which performed quite well).
After we went home I started cleaning the lens and noticed a small curly path inside the lens, which I image could be the scars left from an intense heat beam damaging the lens’s guts, like some coating or other damage-prone surface.
So, is it possible that the immature act to use a lens as burning glass forever damaged the optics inside, or could it be something else?
(Stupidity left aside, it ain't some heavy glass lens, but a third-party prime, and quality seems untouched, so I don't really bother.)
Edit: So here are some pics. Didn't have a macro lens available though:
By looking more closely to it, it could be a small filament of some sorts. It really is inside the lens, I guess between the front and the second element. Impossible to get a precise side view as the image gets distorted by the front element. On the second picture you can even see a rainbow pattern on it as the light gets dispersed.
Also, it's almost invisible. I had to highlight it with a LED light from the side to to have it come out like that (like a scratch in a glass surface would).