Lens Fungus

This is the death knell for most lenses. It is hard to clean off and will often recur. Even if it is successfully removed, the lens elements may be irreversibly damaged. Fungi have been known to etch and blemish the glass coatings. So, even a good cleaning may not restore the lens to its original condition.

Lens fungus may escape detection for a long time because it usually starts from the inner wall of the lens barrel. Even when it infects the glass elements, it starts from the side and grows inwards with a fine mesh-like growth. Image quality will not be affected initially. In fact, unless you look closely with the aperture wide open, you may not notice it.

Eventually, the growth becomes thicker and you will start noticing reduced light entry and increased flaring. In late stages, the growth will become thick, and sharpness and exposure will be affected. The lens is now virtually useless. Although you can still get it cleaned, it is very likely that the fungus would have eaten away the coatings by now.

You can check for lens fungus by flicking the aperture pin near the rear element to keep the aperture wide open. Then shine a bright light through the lens. You can also place the lens under a bright source of light and alternately look through both ends. By varying the distance of the lens from your eyes, you can focus on the different elements in the lens and look for fungus.

Generally, if a used lens has fungus, you should not buy it. But if the owner is putting it out for sale at almost next to nothing and you are confident about getting the lens cleaned up, it may still be worth a try. Just make sure the growth is minimal.

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