Fungus on Lens Elements
Written by Dave Barth, 19 January 2009.
Fungus is bad stuff. It occurs in lenses when damp air gets into them and stays there. Lenses used in coastal areas or places where the humidity is high
are most susceptible to fungus. Every lens has at least one hole or slot in the barrel to allow air to go in and out as the lens is focused or zoomed or the ambient
air pressure changes.
Air pressure changes when a person takes a lens up in a plane, takes the lens up a hill or mountain, or the weather changes from high to low or vice versa.
The cabin pressure in airliners is set to about 2,500 meters (8,000 feet) above sea level.
When a lens is focused or zoomed such that the barrel extends, air gets sucked in. There is usually a filter in the hole on better lenses, but fungus can get past
the filter over a period of time. Better lenses are shipped with desiccant packets that absorb some of the moisture. Of course, if there is too much moisture, the
desiccant saturates and there is still moisture.
Fungus is airborne mold spores that are everywhere. When the fungus gets into a damp, dark place where air doesn't circulate much (like inside a lens), it settles
onto a surface and grows. It usually looks like a white or gray powder on a lens element, but it won't shake off. Fungus has to be wiped off. The trouble is that when
you rub off fungus, if there is a coating on the surface of an element, it often either comes off too or the surface of the coating is pitted.
For mirrored lenses like the 500mm and 1000mm Nikkors, if the fungus is on the mirror, when it is wiped off, the silver may come off or be pitted.
One solution is to disassemble a lens, wipe the fungus off the elements, and either reassemble the lens without recoating the elements or find an expert who can
recoat the elements. A ruined mirror would probably have to be resilvered or be replaced.
Since the work of disassembling a lens, cleaning off the fungus, recoating the optics, and reassembling it is so expensive, it is usually done only with valuable
lenses like the 300mm f2.8 or the 400mm f2.8. Lenses like these cost $5,000 and up, so spending $500 to have them fixed up may be a viable solution.
The 500mm f8 Nikkor lens I owned had two spots of fungus on the mirror, each about half a square inch in area. Repairing that lens wouldn't have been worth the
money. It would have been better to use the lens as it was or to use it for parts. The photos I took with the lens did not show any sign of a problem, but if photos
taken by two different lenses, one with fungus and one without, were compared, an expert could probably see the difference. If the fungus is bad enough, the reduced
quality of the photo will be obvious, and it will be blurry, out of focus, or dark.