Poke around the secondary market for some vintage camera lenses and you’ll quickly find that lens fungus is one of the biggest problems in that market. Mathieu Stern is a guru when it comes to old glass and he has put together a quick video tutorial about how to get that nasty stuff cleaned out.
Fungus contamination happens when a lens is exposed to moisture and dust. It can look like small lines, smudges, or even little cracks at first glance. The problem will continue to worsen if you don’t treat it and it can absolutely affect image quality if not addressed.
This simple washing technique works great for fungus on the front element of a lens, but if the fungus lives deeper inside with the other elements, the process of taking it apart can get very involved, so keep that in mind when buying that super-cheap vintage lens on eBay.