Tried cleaning fungus off a lens element with hydrogen peroxide and a UV light

I had this old 50mm f/1.4 that had some pretty stubborn fungus growth inside, nothing crazy but enough to show up in shots. I kept reading online that a mix of hydrogen peroxide vapor and UV light could kill and clean it off without taking the lens apart, so I figured why not give it a shot. Stuck the lens in a sealed bag with a little dish of 3% hydrogen peroxide, set it under a UV lamp for about 6 hours. Pulled it out and the fungus looked way less visible, maybe 70 percent gone after wiping what I could reach with a lens cloth. But I also noticed some haze on the inner elements that wasn't there before, almost like a residue left behind. So I ended up having to disassemble it anyway to clean that off with proper solution. Lesson learned: it might work for surface stuff, but for sealed internal elements, you're still better off cracking it open. Has anyone else tried this method and gotten better results without the haze?

2 comments

2 Comments

eva_rivera1d ago

That vapor method leaving haze makes total sense now, totally changed my mind.

jackson.wesley1d ago

The UV light part is actually correct but the hydrogen peroxide vapor thing is half baked. Hydrogen peroxide breaks down into water and oxygen when hit with UV, so you basically created a steam cleaning effect inside the lens. That's the haze you saw, it's just the water residue left behind from the reaction. 3% is also way too weak to do anything useful as vapor, you'd need at least 12% to get a real kill rate and even then you risk damaging coatings. I've done this a few times on cheap lenses and the only way it works clean is if you can circulate fresh air through the lens barrel during the process. Sealed bag just traps all that moisture.