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.
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.
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.
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.
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