Lenses are vulnerable to fungal infections which can if left unchecked damage the surface of the lens. A very wide range of fungus’s are responsible, these include the families Phycomycetes, Ascomycetes and Fungi Imperfecti, to name a few.
There are a number of myths often repeated about fungal infection in lenses, such as the belief that they are very contagious, and that once infected the lens is ruined beyond any hope of repair. The fact is that these myths are very largely untrue. While it is certainly true that in certain circumstances a lens can be so badly affected by fungus that it is no longer economic to attempt the repair, in most cases the lens can be ‘cured’ and returned to service without any fear of transmitting the infection.
Some fungi secrete acids, and other substances which will attack the coatings on the lens surface. In extreme cases this etching can penetrate quite deeply into the coating. Some of these secretions are waste by-products of their biology, but often it is part of their way of collecting nutrients. Rectifying damage of this type involves removing the damaged coatings and re-coating the lens. This is rarely economic.
Fortunately, these very corrosive fungi are not the most common fungi to attack lenses, though the fact that many fungi are difficult to clean off the lens surface leads many to believe the opposite is true. Fungus growing into the lens cement is more problematic, as the elements have to be separated in order to clean them, and then re-cemented. Usually a job for a professional.
Because of the great variety of fungi, it is not possible to speak of any particular case and hold it up as a typical example. There are however, a number of things which can be said about all fungi, which permit a general approach which should be effective in most cases.
About Fungi in General
First it will help if we know something about fungi. This will give us an understanding of how to deal with fungal infections, the damage they can cause, and most importantly, how to prevent fungal infections in the first place.
Fungi do not photosynthesise, and cannot manufacture their own sugars, or amino acids from minerals in the way green plants do. Like animals they are completely dependent on other organisms for sources of sustenance. Fungi therefore require a source of organic material to grow and thrive. The fungi do not themselves derive much, if any nourishment from the glass or its coatings, but live mainly off organic contamination found on lens surfaces, such as oil films, or dust, and in the materials used in the construction of lens assembly.
As with most organisms they prefer a varied diet and tend not to thrive on a highly refined diet. In older lenses the fungus penetrates into the ‘Canada balsam’ used to cement lens elements together. Which as a naturally derived material, includes a range of impurities. Modern synthetic cements are less prone to this, as they don’t contain the variety of nutrients.
The fungi themselves consist of fine filaments of fungal cells called hyphae, which form webs of white fibres in the infected materials. These fibres penetrate into the substance of the infected item, where they secrete enzymes and other chemicals to digest the material on which they feed. These secretions, and the byproducts represent the corrosive agents which cause the damage in some infections.
Mushrooms and toadstools, with which we are all familiar are in fact only a tiny part of the particular fungi that produce them, and represent only the ‘fruiting bodies’ by means of which they propagate themselves. The fungus that produces the visible mushroom can be quite literally massive in extent. Often a single fungus can extend across several acres underground!
Fungi propagate by means of spores. These are commonly dispersed by means of the fungus’s fruiting bodies, which in many cases are quite small. These spores are microscopic, and produced quite literally in their billions. These are in the air around us all the time, and will germinate anywhere they find the right conditions. Because of their small size and large numbers they will find their way into almost any ventilated space.
Fungi also require a certain amount of moisture in order to grow. Some fungi once established, such as the dreaded ‘dry rot’ in buildings, can synthesise the water they require from the oxygen in the air, and hydrogen found in the attacked organic material. No species however has spores which have this ability, and so fungal spores cannot germinate without moisture. In wood, this moisture level is about 15% by weight, which is rather on the damp side, and there is no reason to suspect that the fungus in lenses is any different.
The fact that the spores are so invasive has led many to assume that this is an indication of their infectious potential, and many lens repairers will have nothing to do with infected lenses lest the spores they carry infect other lenses in their workshop. This is probably a futile exercise unless they operate in ‘clean room’ conditions, as otherwise the workshop likely contains millions of fungal spores already. The reason that they are not already thick with fungus has more to do with moisture, or the lack of it, than the presence or otherwise, of fungal spores.
In effect, all lenses are probably already infected, and spreading of the infection from one lens to another is not the major cause of infestations. It’s poor storage conditions that have to be avoided, so the the spores cannot germinate.
As always prevention is better than cure.
What we now know about fungi, tells us that if we want to avoid fungal damage, we should keep our lenses clean and dry. So don’t keep your lenses in your kit bag, or in the garage, and always dry bags and cases thoroughly if they have become damp. (From rain for example.) Clean the outside of your lenses regularly to remove dust deposits, and don’t allow greasy deposits to remain on the lens surfaces should this occur.
Some photographers will tell you that they never clean their lenses unless absolutely necessary for fear of scratching the coatings. My own view is it’s probably best not to get them dirty in the first place, and to check regularly to make sure that they are clean.
We should also keep them well away from possible sources of nutrients. This probably means that you shouldn’t be keeping your sandwiches in the kit bag either. In fact the moisture from your sandwich would probably be enough to worry about in any case. Keep all kit bags and cases scrupulously clean.
It also tells us that attempting to keep fungal spores out of our lenses is probably a futile exercise. Given that there are plenty of places spores can get into most lenses, there is a very good chance that the lens body already contains sufficient spores just waiting for the right conditions.
Tests have demonstrated that almost any lens will begin to grow fungus within weeks (5 days in one instance!) of exposure to damp, dark, warm conditions. So avoid such conditions whenever possible.
The good news is that drying a lens out thoroughly, and keeping it dry will effectively stop the fungal growth in the majority of cases, and will prevent any spores from germinating.
Killing Fungal Infections of Lenses
Killing fungal infections can be difficult, especially if you want to make sure that you have killed off all the hyphae, and that any spores are also killed. This means that any treatment should be applied to the lens as a whole, otherwise live spores may remain ready to re-infect the lens as soon as the opportunity arises.
Finding a suitable chemical agent that can be applied to all the lens components, preferably without entailing the disassembly of all the assemblies is something of a challenge. Ideally the fungicide should have low toxicity, easy to use, penetrate all parts of the assembly, and should not damage the materials used in the lens construction. Ideally then, a fumigant of some sort is indicated.
Thymol, found in plants, is a naturally occurring anti fungal. In its pure form it consists of a white crystalline substance, and is used in medical preparations as a preservative. It has also been experimented with in connection with preserving optical instruments with some success. It is available in fumigant form for use by beekeepers who use it to kill parasitic mites which infest their hives, and can be most easily obtained in pure from Indian herbal medicine suppliers. I have not personally experimented with fumigation of affected lenses, but it is something I intend to try.
It occurs in herb thyme, oregano, and in high concentrations in the Indian spice Carom (Also known as Ajowan), so maybe a sachet of these in the lens case might help guard against fungal infestation. At very least it will give it a pleasant odour!
Acetic acid in the form of strong spirit vinegar, has been used as an antiseptic for treating wounds, (Remember Jack & Jill?) and also has good anti fungal properties. So soaking in vinegar may prove effective.
Hydrogen peroxide has also been recommended (By Leitz, and others.) as a means of killing fungus, especially when combined with ammonia to act as a cleaning agent. I don’t see this as a practical way of cleaning the whole lens assembly, but should be used to clean the lens elements themselves.
Exposure to strong sunlight, or hard UV has also been recommended, and some advocate the use of x-rays to penetrate deeply into the lens and kill fungal spores. (Maybe if I broke a leg…..)
However, as we have already noted, it may not be necessary to kill the fungal spores, as they cannot germinate without moisture. I suspect that recurring infestations have more to do with fungal hyphae remaining in the lens, and failure to keep the lens in suitably dry conditions.
Practical Methods for Cleaning Fungus From Lenses
Cleaning fungus from the lens is a delicate operation, and not always successful. If a fungus has etched into the coating, the only cure is re-polishing and re-coating the lens elements. This is a highly specialized job, and is unlikely to be an economic proposition for any but the most valuable lenses. In any case, as pointed out, many repairers won’t touch lenses with fungal damage for fear of infesting their workshops. (This is probably a little paranoid, but one can hardly blame them.)
Neither is it possible to make an accurate assessment of the extent of damage until the lens is dismantled. Most fungi cause little or no damage to the lens surface, but as mentioned earlier, others are very aggressive, and they are nigh impossible to tell apart by visual examination.
The lens must first be disassembled to asses the extent of the infestation. Fungus growing in between cemented elements often means that the lens is not economically repairable. Lenses cemented with Canada balsam can be separated by the careful application of heat to soften the cement, but cementing them back together properly is a specialist job. Surface fungi can often be cleaned off and the lens returned to service. The lens may contain fungal spores however, and it will be almost impossible to remove hyphae which have penetrated into the waxes and glue often found in lenses, which will start to grow if given the chance.
One problem is that even if the fungus is dead, and is itself completely removed, they can leave a deposit behind which is very firmly attached to the lens. This is often mistaken for etching damage.
Hydrogen peroxide, or bleach, can be used to kill fungus. According to Leitz, a fungus treatment of 94% distilled water, 4% clear ammonia, and 2% hydrogen peroxide should be used. The lens element should be soaked in the mixture for an hour or more before cleaning.
The hydrogen peroxide is included in order to destroy any fungal spores, and hyphen but unless the whole of the lens including the housings etc. is treated, it is difficult to see that much benefit will be gained. It does however make an effective cleaner.
Many report that household spirit vinegar can be effective for lens cleaning. Given that it has marked anti fungal properties, and easily obtainable I would recommend giving it a try.
Other reports state that ordinary cold-cream is also effective. I have no idea how that works, and I haven’t tried it myself!
As a last resort, a very light swabbing with a soft metal, polish can remove fungus from glass surfaces, quite effectively. Don’t use a polish designed for a hard metal, such as might be used for chrome, these often contain very hard abrasive materials which could remove the coating. (The same may be true of some glass polishes!) I have found silver polish to be extremely effective, cutting the fungus quite effectively with no apparent damage to the coating. (This would be noticeable as a color shift in most coatings.)
If done carefully the most fungus can be removed without also damaging the lens coatings, and properly cleaned and thoroughly dried, there is no reason that a lens so treated should not last for many more years. Even if there is a slight risk of damaging the lens surfaces, the alternative is usually consigning the lens to the scrap bin.
Lenses can be protected from fungal infections by keeping them in dry clean conditions. The paranoia associated with fungal infection of lenses is largely unfounded, and most cases can be treated with a great degree of success. Spreading of fungal infection between lenses is not a major problem, and is very unlikely to be a cause of fungal infestation. There is usually no need to dispose of an affected lens.