Camera lens fungus

What it is and how it looks

Fungus is a living organism, much like a mushroom, and it spreads by spores, which work in the way a plant's seeds do. It feeds by giving off acids, and on a lens those acids can eat into the coatings and the glass elements themselves. The coatings are the thin layers on the glass that cut reflections, and the elements are the individual pieces of glass inside.

It comes in several forms. Thin threads or a spider's web are the plainest sign. Others look like a general haze or clouding, a scatter of specks like dust, or a patch like the mould on old bread. If a speck has a halo around it when you tilt the lens towards a light, treat it as fungus until shown otherwise.

How it gets in

It is rarely the owner's fault. Spores are everywhere, and some may have been inside a lens since the day it was made. Over the years they find their way in as a lens is zoomed and focused, and a lens from 1970 has been used a great many times since.

Where it was kept matters most. Fungus likes darkness and damp, which makes a loft, a back cupboard, a shed or a garage about the worst place to keep an old lens.

What it does to the pictures

That depends on how much there is and where it sits. A mild case may do little, though it can lower contrast and cause flare, which is a wash of light across the picture. A severe case acts like a very dirty lens, with soft patches, flat colour and sometimes part of the image blocked. If the glass has been etched, the picture will be less sharp whatever you do.

If you want to know for yourself, run a cheap roll of film through a camera with the lens on it, or fit it to a digital camera. Fungus is alive, so a lens that is fine today may be affected later.