How do you deal with fungus in a lens

Robert Ley · Apr 16, 2025

I'm new to lens repair and have some general questions on fungus.

Do you segregate the lenses with fungus, is it easy to infect other lenses with the fungus.

There are quite a few articles on cleaning lenses of fungus so that is not an issue.

I have a UV light box that I use for alt printing, can a place the lens inside the UV box and will the UV light kill the fungus?

If the UV will kill the fungus, should I do that before I open the lens?

Thanks for any "light" you can shed on this topic

ic-racer · Apr 16, 2025

I just wipe it off and put the lens back together. Nothing special. Fungal spores are everywhere, there is nothing you can do about it. Don't let them germinate; avoid dark moist storage.

Sirius Glass · Apr 16, 2025

Putting a lens in direct sunlight, preferably with the sunlight shinning into the lens will help kill or reduce fungus.

Robert Ley · Apr 16, 2025

I have never had a problem with any of my lenses as I am aware of how lenses should be stored in low relative humidity and I have maintained that low humidity in lens storage. Have you had any experience with UV light and fungus?

xkaes · Apr 16, 2025

Try sunlight. Just because I haven't had any luck doesn't mean you won't. Maybe it will kill it, but it won't make it disappear. It will still be there -- even if you take it apart and try to clean it.

The first think to consider is "How bad is the problem?". Oftentimes the fungus is only on the far edge and does not impact the image. If that is the case, try to kill it will lots of sunlight and store it in a dry place away from other lenses.

MattKing · Apr 16, 2025

Sirius Glass said:
Putting a lens in direct sunlight, preferably with the sunlight shinning into the lens will help kill or reduce fungus.

If you do this, the UV (in particular) in the light assists with the fungus, but you need to be cautious about heat build up

Andreas Thaler · Apr 16, 2025

Robert Ley said:
Do you segregate the lenses with fungus, is it easy to infect other lenses with the fungus.

That's a well-known horror story from the internet that never ends

Fungus isn't a virus; it doesn't infect.

Dry, cool, and bright conditions keep it away; its spores are everywhere.

Robert Ley · Apr 16, 2025

Andreas Thaler said:
That's a well-known horror story from the internet that never ends

Fungus isn't a virus; it doesn't infect.

Dry, cool, and bright conditions keep it away; its spores are everywhere.

Thanks for clearing that up for me. I have never had to deal with lens fungus, but I am learning.

RalphLambrecht · Apr 16, 2025

ic-racer said:
I just wipe it off and put the lens back together. Nothing special. Fungal spores are everywhere, there is nothing you can do about it. Don't let them germinate; avoid dark moist storage.

I fully agree. Putting you lens on a sunny window sill may be enough to kill the fungus. If you can get to the fungus, clean it with hydrogen peroxide (15%) followed by naphtha and/or alcohol(>90%). If your storage environment is too humid, consider a dehumidifier.

campy51 · Apr 16, 2025

What do you do if it's etched in the glass?

Andreas Thaler · Apr 17, 2025

RalphLambrecht said:
I fully agree. Putting you lens on a sunny window sill may be enough to kill the fungus.

You shouldn't rely on this, as glass absorbs some UV light. I also wouldn't leave a lens in the sun for long periods of time, as this can affect the (old) lubrication.

mshchem · Apr 17, 2025

I get rid of stuff like this. My storage conditions are such I've never had a problem. I've been given some old enlarging lenses, I've re-gifted or donated.

All about the dew point (humidity)

BradS · Apr 17, 2025

I live in a warm and dry climate where fungus isn't an issue but in the past, if I happened to acquire a lens with a bit of fungus, I generally just ignored it and carry on. If it is bad, the lens went out with the garbage. Do the community a favor and take that crap out of circulation.

jonny88 · Apr 17, 2025

I’d keep any fungusy lens in its own bag or box so spores don’t float onto your other glass. UV light might zap some on the surface, but it won’t reach deep between elements-so you’ll still need to open it up and clean it (a mix of isopropyl alcohol and a little ammonia usually does the trick)

250swb · Apr 18, 2025

A bit of fungus around the edge won't affect the image quality so kill it to stop it's spreading or look into how you keep your lenses because humidity and darkness will do for your other lenses as well. As for putting a lens into quarantine it doesn't work, fungus spores are all around us, we are all breathing them in right now and opening your drawer to take a lens out lets fungus spores in. The very best antidote is to use the lens and let it have plenty of UV exposure.

koraks · Apr 18, 2025

Sirius Glass said:
Putting a lens in direct sunlight, preferably with the sunlight shinning into the lens will help kill or reduce fungus.

Not really. The UV might kill some of the fungus, but the main advantageous effect would be the dissipation of moisture that the fungus needs to grow. Put the lens back into a humid environment and the fungus will happily continue to spread.
Also, UV does NOT 'reduce' fungus - it won't remove existing fungal growth. The theory is that it'll kill the fungal growth, but given its resilience, it's unrealistic to expect you'll ever do a complete job at it. You may slow it down temporarily.

Mechanical & chemical removal is the only real way to deal with fungus, combined with storing the lenses in low-humidity environments to prevent the problem. Note that over time, fungus etches the glass and the coatings, so in bad cases, after removing the fungus itself there will still be permanent and irreparable damage to the glass.

Andreas Thaler · Apr 18, 2025

koraks said:
Mechanical & chemical removal is the only real way to deal with fungus, combined with storing the lenses in low-humidity environments to prevent the problem. Note that over time, fungus etches the glass and the coatings, so in bad cases, after removing the fungus itself there will still be permanent and irreparable damage to the glass.

That's why I wouldn't delay removing fungus until it's caused any damage.

However, a single small fungus star that may have been sitting there starving to death for decades wouldn't be a reason for me to risk opening the lens.

psmithp · May 5, 2025

Many years ago, when I lived in the central African rainforest (more or less, in Yaounde and LIbreville), I kept my lenses fungus-free in a cupboard heated by two 100W electric bulbs (that was then) wired in series, giving a resulting power output of 25W, most of it as heat. The heat was not a problem although you normally had enough of it around you, the higher temperature (relative to the surroundings) lowered the relative humidity until below the fungus' comfort zone.
Today you would have to get a proper heating element, they are often used in electrical switchboards.

cowanw · May 5, 2025

The Fungus we talk about has two important states, the spore and the branching mycelium. Sunlight (UV) and other sterilizing situations will kill mycelium, which sits dead unless cleaned away.
Spores are not killed and will sit harmlessly until growth conditions are favourable.
Only a few types of fungi eat coatings. Most of the Fungi (cheese, peaches grapes) in your house have no relation to you photo equipment.
Reasonable light, reasonable humidity and reasonable heat are all that is required.
Cleaning away mycelia from the lenses is a personal decision.
If you see fungus it is always due to bad storage; which might be associated with other bad decisions and usage that might impact your decision to acquire a particular piece of equipment.

How do you deal with fungus in a lens

Robert Ley

ic-racer

Sirius Glass

Robert Ley

xkaes

MattKing

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Andreas Thaler

Robert Ley

RalphLambrecht

campy51

Andreas Thaler

mshchem

BradS

jonny88

250swb

koraks

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Andreas Thaler

psmithp

cowanw

How do you deal with fungus in a lens

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