fungus

[geotrupede]

Hi,
I would like to find out more about lens fungus.
I have been finding a lot of information on the internet but I am not so sure it is really reliable. So I am here in the best forum to ask the experts to share some tips and shine some light through...

What I am interested is:

-1- speed of growth
-2- dead fungus, how can you tell from a living one
-3- can be cleaned with UV/sun rays or does it need special facilities (Leica uses a tropical decease specialised clinic in Hamburg...)
-4- when cleaned, can it come back?

Please reply if you have first person experience.
Thanks
G.

 

[Leigh]

In a photographic context, fungus lives on the anti-reflection lens coatings.

After digesting the coatings plus any grease and oil, from fingerprints and such, fungus excretes Hydrofluoric Acid, which etches glass.

The main problem here is if you can see the fungus, particularly the tendrils radiating from a central point, it's already too late.
The acid has already etched the glass, and the lens is toast.

If you have such a lens, IMMEDIATELY enclose it in a zip-lock freezer bag and keep it away from other lenses.
Fungus spreads by spores through the air, and will land on and "infect" any surface it finds, particularly other lenses.

The NIH site recommends use of a 50/50 mix of Hydrogen Peroxide (H2O2) and Ammonia (NH3) if a proper fungicide is not available.

Here's a link to the entire page, which discusses fungus on optical elements:
Fungus: how to prevent growth and remove it from optical components

I've received any number of lenses for repair that had fungus damage.
In every case I returned them as being unrepairable, since the acid etches the glass, and that cannot be reversed.

- Leigh

 

[geotrupede]

What to say... I knew it was the right place to ask
Thanks for the link! It is excellent.

If a lens has a small and isolated fungus (like 1 - 2 mm), how long is this going to take to cover the whole surface? Is speed of growth a function of humidity?
So, for instance, in a dry environment the growth could be stopped?

And in general, am I correct that a lens with fungus is something to stay away if possible?

Thanks
G>

 

[Leigh]

If there is a small, isolated infestation, and you have completely cleaned it such that no fungus remains on the lens, you can mitigate the damage by filling the trace with black india ink and allowing it to dry. That will prevent transmission and reflection of light.

I should emphasize that this generally only works for very small areas.

Speed of growth depends on many factors, some of which cannot be assessed.
Fungus is like invaders from a galaxy far far away. It must be totally annihilated when first found, or it will rule the planet.

"Using" a lens that has fungus is not an option, regardless of how you define "use".

You must kill all fungus immediately. Fungus spreads by emitting spores, which you can't see.
An lens that has not been thoroughly cleaned by killing and removing ALL fungus, must be isolated as I mentioned above.

When I tell a customer that he needs to toss a multi-thousand $$ lens because it can't be repaired, and he does so ...
That means this is a real problem, serious as a heart attack.

AFAIK, no lens manufacturer will accept a fungused lens for re-manufacture, due to the danger of infecting new stock.

Note: A lens that has sustained etch damage cannot simply be re-ground, since that changes its dimensions.

- Leigh