Friday 17 May 2013

Lens Fungus


A lot of utter rubbish appears on internet fora — interspersed with accurate information — on the problem of fungus growing on camera lenses. The advice usually falls into three categories: prevention; whether removal is possible; effects on image quality.

In this post I only deal with the last aspect since we read comments that say the lens must be degraded while others assert there is no effect. Very few of those who offer an opinion take the effort to consider whether any tests they have done are valid in relation to where the fungal hyphae were situated on the lens in question.

In most cases, in old lenses, fungal growths start from the edge of the front element. That is because fungal spores present in the air need an organic source of carbon  — plus warmth and humidity — on which to develop. The edge of a lens, under the rim of metal or plastic, is an ideal resting place for bits of cloth, tissue, skin particles and dust that provide a source of carbon. I have also read that some of the paints and balsams used in lens manufacture also provide a nutritious surface for fungal growth. Fungal hyphae — again in warm and damp conditions — then spread out across the surface of the lens in search of more food. They even release chemicals which etch the coating and glass.

So, first of all, we have to distinguish between between fungal growth around the edges and that which has spread to reach the centre. I am only dealing with the former since the image must be degraded if the whole lens is covered. I am also only dealing with fungus on the front element of the lens. The calculations needed for fungus on inner elements, particularly in zoom lenses, are more complex.

What degradation should we expect in a case of fungus around the edges? What testing would be effective in deciding?

I am going to work through an example to demonstrate how decisions on whether image quality would be affected at a particular exposure could be taken. The photograph below shows fungus on the front element of an Exakta Pancolar 50 mm f/2 lens.

Fungal hyphae on the front element of an Exakta Pancolar 50 mm f/2 lens

The diameter of the front element of the lens shown is 29 mm and a quick measurement showed that the central area clear of fungus is 22 mm in diameter.

Whether that outer region of the lens infested by fungus would have any effect on the final image depends on how much of the lens area is used in a particular exposure. At the maximum aperture for this lens of f/2, the whole diameter of the lens, including that part bearing fungus, is used for the exposure. Therefore, at maximum aperture we would expect the image to be degraded by fungus on the lens. But at which aperture would the affected outer region of the lens not be used and become safe to use for an undegraded image— f/2.8, f/4, f5.6...f/22? Clearly, we need to consider what diameter of the front element is used at each aperture setting.

The diameter of the lens aperture (set by a diaphragm between the front and back elements) is easy to calculate. The diameter (in mm) equals the focal length (50 mm in this case) divided by the f/ number. Thus, the diameter of the aperture (not the diameter of the front element) of a 50 mm lens at f/2 is 25 mm, at f/4 12.5 mm etc.

I know I am teaching my grandmother to suck eggs, when I draw attention to the fact that the change in diameter is not linear with changing f/number. The change in diameter is much greater between f/2 and f2.8 (a halving of exposure) than it is between, say f/8 and f/11 (also a halving of exposure). The following diagram shows how the diameter of the aperture changes with f/number for a 50 mm lens.

Diameter of the diaphragm at different f/numbers

Because light rays are still converging after they pass the front element on their way to the diaphragm, the diameter of the front element is larger than the aperture. I have measured a number of lenses including the Pancolar and the maximum diameter of the front element is about 1.1 times the diameter of the maximum aperture, that is 29 mm compared with 25 mm. So, we need to multiply the diameter of the aperture at each f/ number by 1.1 to map onto the front of the lens the actual diameter of the front element used at each f/ number setting.

This I have done in the following diagram. It shows how much of the front element of this lens is used at each f/number, from f/2 to f/16.

Diagram showing the area of the front element of the lens used as different f/numbers.
Also shown is the diameter of the fungus-free central area compared
with the diameter of the area used at f/2 and f/2.8


The take-home message is completely clear: there can be no effect of the fungus from f/numbers larger (i.e. apertures smaller) than f/2.8. In other words, even though the lens appears visually to be badly affected, it can be used as a lens with a maximum aperture of f2.8 without fear of degradation of the image.

The same approach can be used for other lenses, although it is easier with those of fixed focal length. All you need to measure is: the maximum diameter of the front element compared with the calculated diameter of the diaphragm at maximum aperture (calculated from focal length and f/number); the diameter of the lens unaffected by fungus.

Finally, returning to this 50 mm Pancolar lens, I would need to test it at f/2 to see if the theoretical degradation caused by light scattering in and under the fungal hyphae was actually discernible in practice. Testing at any other f/number other than within the range f/2-f/2.8 would be pointless.




No comments:

Post a Comment