There is a particular sinking feeling that comes with holding a lens up to a window and seeing thin white threads spreading across the glass. Fungus. In Bangladesh it is not a rare accident. It is the default outcome for any lens that gets left in a camera bag and forgotten for a few months.
Dhaka sits above 75 percent relative humidity for a large part of the year, and during the monsoon it regularly climbs past 85. Mould spores are in the air everywhere, all the time. Your lens does not need to get wet for fungus to take hold. It only needs to sit still, in the dark, in humid air, which is exactly what a camera bag in a corner of a flat provides.
The good news is that prevention is cheap and takes about ten minutes a week. Repair is neither. This guide covers how to keep a Sigma lens healthy through a Bangladeshi year, and what your options are if fungus has already started.
What Lens Fungus Actually Is
Fungus inside a lens is a living organism growing on the internal glass surfaces. It feeds on the microscopic organic residue that ends up in there over time: skin oils, dust, traces from lubricants and adhesives inside the barrel. Given moisture and darkness, spores that are already floating in the air settle, germinate and spread.
Under a light it looks like a spider web, or sometimes like fine frost crawling out from the edge of an element. Early on it does very little to your images. Left alone it spreads, scatters light and reduces contrast, and eventually the acids it produces etch permanently into the lens coating. At that point cleaning removes the fungus but leaves the damage behind.
Fungus can also spread. Store an affected lens in the same sealed bag as a clean one and you have doubled your problem.
Three Conditions Fungus Needs
Fungus needs moisture, darkness and stillness. Remove any one of them and growth slows dramatically. Remove two and it effectively stops.
- Moisture: relative humidity above roughly 60 percent is the danger zone. Below 45 percent, growth stalls.
- Darkness: ultraviolet light kills spores. A lens that sees daylight regularly is much less likely to develop fungus.
- Stillness: stagnant air lets spores settle. Airflow and regular handling disrupt them.
This is why the photographer who shoots every weekend rarely has fungus problems, while the person who bought a lens for a wedding and put it away for eight months opens the bag to a disaster. Use is protection.
The Number That Matters: 40 to 50 Percent
Aim to store your lenses at 40 to 50 percent relative humidity. That range is dry enough to stop fungal growth and still wet enough that rubber seals, focus ring grips and lens coatings do not dry out and crack. Going far below 35 percent for long periods is its own kind of damage, so drier is not automatically better.
You cannot judge humidity by feel. Buy a small digital hygrometer. They cost very little in Dhaka electronics markets and they turn a guess into a number you can act on. Put one in whatever container your lenses live in and check it once a week.
Storage: Three Options, in Order of How Well They Work
1. An electronic dry cabinet
This is the correct answer if you own more than one lens or a single expensive one. A dry cabinet is a sealed cabinet with a small dehumidifying unit that holds a set humidity level continuously. You set it to 45 percent, plug it in, and stop thinking about the problem.
Running cost is negligible, roughly what a low wattage bulb consumes. The cabinet also protects against dust and casual damage. Set against the cost of a 150-600mm telephoto, or even a single 35mm Art lens, a dry cabinet pays for itself the first time it prevents a repair. If you shoot professionally in Bangladesh, treat it as required equipment rather than an accessory.
Buy one sized for your gear plus room to grow. Everyone underestimates this.
2. Airtight box with silica gel
The practical option if a cabinet is out of reach right now. Use a genuinely airtight plastic container, not a camera bag, not a cardboard box. Add rechargeable silica gel, the kind with colour indicating beads that turn from orange to green or blue to pink when saturated. Put a hygrometer inside so you actually know what is happening.
The catch is that this needs maintenance. During the monsoon the gel can saturate within a week or two. Recharge it by heating it in an oven or drying it in strong sunlight, following the packet instructions, and rotate two batches so one is always ready. Silica gel that has stopped working looks identical to silica gel that is working, which is why the colour indicating type is worth the extra cost.
3. What not to do
- Do not store lenses in the padded camera bag long term. The foam holds moisture against the lens, and it is dark and still inside.
- Do not use uncooked rice as a desiccant. It is barely effective and it sheds starch dust into your gear.
- Do not leave gear in a car. Heat plus humidity plus condensation is the fastest route to a problem.
- Do not store lenses against an exterior wall or near a bathroom, both of which stay damp in Bangladeshi buildings.
- Do not seal a lens in a plastic bag with no desiccant. You have trapped humid air, not excluded it.
Habits That Keep Fungus Out
Storage does most of the work, but a few small routines close the remaining gaps.
- Use your lenses. Even taking a lens out for twenty minutes of daylight shooting every couple of weeks helps.
- Wipe the barrel and the rear mount with a dry microfibre cloth after every shoot. Skin oil on the outside eventually finds its way inside.
- Keep both caps on and keep the mount clean, but let the lens air out for an hour before it goes into storage after a humid day.
- Keep a UV or protective filter on the front element for field use. It takes the fingerprints and the salt spray instead of your front glass.
- Isolate any lens you suspect has fungus. Do not put it back with the healthy ones.
- Buy used lenses carefully. Always inspect against a bright light before paying.
Monsoon and Field Protection
Bangladesh gives you roughly four months where rain is a daily possibility. Shooting through it is entirely doable with a bit of care.
Carry a rain cover. A purpose made camera rain sleeve costs very little and folds into a pocket. A large clear plastic bag with a hole for the lens hood works in an emergency and has saved more shoots than anyone admits. Keep the lens hood fitted at all times in rain, since it keeps drops off the front element far better than you would expect.
Condensation is the underrated danger. Walk out of an air conditioned room into Dhaka’s outdoor air in July and moisture will immediately fog your lens, inside and out. The internal fogging is the part that matters, because that is water sitting on internal glass in exactly the conditions fungus wants. Prevent it by letting the gear acclimatise: put the camera and lens in a sealed bag before you leave the cold room, and only open it once the bag has reached outdoor temperature. Twenty minutes usually does it. If a lens does fog internally, do not shoot with it. Let it sit somewhere warm and dry until it clears completely.
Near the coast, at Cox’s Bazar or on a boat in the Sundarbans, salt spray is corrosive to metal parts and coatings. Wipe the lens down with a slightly damp cloth and then a dry one at the end of the day. Salt left overnight causes real damage.
Weather sealing on Sigma’s Sports and Art lenses helps considerably with splashes and light rain, but it is water resistance, not waterproofing, and it does nothing at all about ambient humidity. A sealed lens stored badly will grow fungus exactly like an unsealed one.
A Cleaning Routine That Will Not Damage Anything
Clean the glass as little as you can get away with. Every wipe carries a small risk to the coating, so the ideal frequency is only when there is actually something on the lens.
| Step | Tool | How |
|---|---|---|
| 1 | Air blower | Blow off loose dust first, never compressed air cans |
| 2 | Soft lens brush | Sweep remaining particles from the centre outward |
| 3 | Microfibre cloth | Light circular motion, no pressure |
| 4 | Lens cleaning fluid | Apply to the cloth, never directly onto the glass |
| 5 | Dry side of cloth | Finish to remove any streaks |
Things to keep away from your lenses: tissue paper and newspaper, both of which are abrasive; shirt sleeves, which carry grit; household glass cleaner, which contains ammonia and will strip coatings; and your breath, which puts warm moisture onto glass in a country where you are trying to do the opposite. Wash microfibre cloths occasionally without fabric softener, and replace them when they stop feeling soft.
If You Already Have Fungus
Check every lens you own tonight. Shine a torch through from the front at an angle and look through from the rear. Fungus shows up as fine white or grey threads, often starting near the edge of an element. Haze or a milky appearance can also indicate early growth.
| Stage | What You See | What To Do |
|---|---|---|
| Very early | A few faint threads, no image effect | Move to 40-45% storage and expose to daylight regularly |
| Established | Visible web across an element | Professional service disassembly and cleaning |
| Advanced | Large growth, visible haze in photos | Service, expect some coating etching to remain |
| Severe | Etched coating, permanent flare and low contrast | Element replacement or accept the loss |
Do not attempt to open a modern Sigma lens yourself. These are precision assemblies with stabilisation units, electronic contacts and elements aligned to tolerances you cannot restore on a table at home. Opening one voids your warranty and usually turns a repairable lens into a paperweight.
Take it to authorised Sigma service support in Bangladesh instead. A technician can disassemble, clean the affected elements and reassemble with correct alignment. Whether it is worth doing depends on the lens. On an expensive Art or Sports lens, almost always. On an older budget lens, the service cost may approach replacement cost, and that is a judgement call.
One thing to be clear about: fungus damage is not covered under manufacturer warranty anywhere in the world, because it results from storage conditions rather than a manufacturing defect. Prevention is the only free option you have.
A Seasonal Checklist for Bangladesh
| Season | Main Risk | What To Do |
|---|---|---|
| Monsoon, Jun to Sep | Peak humidity and rain | Check silica gel weekly, rain cover in the bag, watch for condensation |
| Autumn, Oct to Nov | Humidity still high | Full inspection of every lens, deep clean, recharge desiccant |
| Winter, Dec to Feb | Dust and morning fog | Blower after every shoot, keep filters on, dust off storage seals |
| Summer, Mar to May | Heat and car storage | Never leave gear in vehicles, keep the cabinet away from direct sun |
Frequently Asked Questions
Direct sunlight slows and can kill surface spores, and it is a reasonable free measure for very early growth. It will not clear established fungus inside the lens, and you should never leave a lens pointed at the sun, since the elements can focus enough heat to damage internal parts.
During monsoon, check weekly and expect to recharge every one to three weeks. In winter it lasts considerably longer. Colour indicating beads take the guesswork out of it.
Regular use genuinely lowers the risk, but the monsoon here is severe enough that I would still recommend one for anything valuable. If you own a single kit lens, a good airtight box with silica gel is a reasonable compromise. If you own a 24-70mm Art or a long telephoto, get the cabinet.
No. Sealing keeps out splashes, dust and light rain during a shoot. It does not control the humidity of the air already inside the lens or the air it is stored in.
Yes. It will not stop on its own in this climate, and cleaning early is cheaper than cleaning late. More importantly, early cleaning happens before the coating gets etched, which is the damage that cannot be undone.
Final Thoughts
A dry cabinet, a hygrometer and a ten minute weekly habit will outlast several camera bodies. The photographers in Bangladesh who never seem to have fungus problems are not lucky. They just decided early that storage was part of owning a lens rather than an optional extra.
If you are buying a Sigma lens, budget for storage in the same conversation. And if you already own one that has been sitting in a bag since last year, go and check it with a torch before you finish reading this.
