Lens Fungus and Storing Optics in the Tropics
Fungus is the slow cost of humid air. It grows inside lenses and binoculars kept in still, damp, dark places, and the marks it leaves on the glass do not come off. This lesson shows what it needs, how to store optics so it never starts, and what to check when you buy.
Dew Risk Calculator · Dew Heaters and the Air-Conditioned Room
"Why does fungus form in lenses, and how do I get rid of it?" has been viewed more than 160,000 times on one question-and-answer site. For people in the tropics it is not a rare worry: the air most of the year is the air fungus likes.
What fungus needs
ZEISS lists what lets fungus spores, which are everywhere, start to grow:
- relative humidity of at least 70% for more than three days;
- little or no airflow;
- darkness;
- something to feed on: textile lint, traces of grease, varnish, dust and dirt;
- temperatures between 10 and 35 °C.
Guyana supplies most of that by itself. Averaged over ten years, night humidity at Georgetown runs from about 84% at 19:00 to about 89% by 04:00, and the temperature stays inside that band all year. A camera bag in a closed cupboard adds the darkness and the still air.
How fast it happens depends on the humidity. The Canadian Conservation Institute gives about 100 days before mould shows at 70% humidity, about 10 days at 80%, and about two days at 90%. It also notes that any spell below 55% stops growth.
At a steady 80% humidity, about how long does mould take to appear, by the Canadian Conservation Institute's figures?
- Two days
- About ten days (the right answer)
- About a hundred days
- It never grows at 80%
The Institute's table gives about 10 days at 80% humidity, against about 100 days at 70% and two days at 90%. Humid nights shorten the clock quickly.
Prevention matters more than cleaning. ZEISS warns that the fungus's by-products, such as acids, damage lens surfaces for good, so even a cleaned lens can keep the etched marks.
How to store optics
The goal is to keep the air around the glass dry and moving, without drying it to the bone. ZEISS says to keep the humidity below 60% and never under 30%, which can harm the instrument. Nikon's camera manual says the same upper limit: avoid places over 60%. A target of 40 to 55% sits inside both limits and below the 55% the Institute gives for stopping growth; that target is our reading of the three sources.
- A dry cabinet: a cabinet with a built-in dehumidifier and a hygrometer that shows the humidity inside.
- A sealed box with silica gel: a plastic food box with a good seal, a few silica gel packs and a small hygrometer. Dry or replace the gel when its colour shows it is full.
- Not leather, cloth or wood: ZEISS names all three as poor containers, because they hold moisture and feed fungus.
- No mothballs: Nikon warns against storing cameras with naphtha or camphor.
- Light and air: fungus likes the dark and still air. ZEISS adds that short spells of sunlight or UV light may help. Take your gear out and use it.
- Dry before you store: after a dewy night, let the gear dry with its caps off before it goes into the box, so you do not seal the night's water in with it.
Which storage humidity do ZEISS and Nikon both support?
- 80 to 90%, the same as the night air
- 65 to 70%
- 40 to 55% (the right answer)
- 10 to 20%, as dry as possible
Both say to stay under 60%, and ZEISS warns against going under 30%. 40 to 55% is inside that band and below the 55% at which mould stops growing.
When you buy or borrow
- Shine a torch through it: hold a small torch behind the lens or eyepiece and look through from the front. Fungus shows as fine threads or branching webs inside the glass, not loose dust.
- Ask about the warranty: some sellers do not cover fungus, so ask before you buy, and keep the box and the receipt.
- Bring it home dry: a new lens or telescope goes straight into the dry box, not the cupboard.
If you find fungus, do not open the lens yourself. A repair service can sometimes clean it, but any marks already etched into the glass stay.
Ten years of Georgetown and Lethem nights: how often the dew rule reads high, and how humidity climbs through the night.