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I just saw these blue/purple spots underneath my lens (attached here). This is a Nikon D90 which has the Nikkor 18-105 mm lens mounted on it. I haven't used it for maybe 2 months now, and I kept it inside my wardrobe in the camera bag.

I'm wondering what caused this to the lens. Should I be worried? Is there any way to clean it up? Will this spread inside the lens?

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  • \$\begingroup\$ Additionally if it is fungus (and I cant think of anything else that it could be) make sure for the future to keep your lenses dry; e.g. by stocking up on silica gel \$\endgroup\$
    – PlasmaHH
    Mar 15, 2015 at 20:39

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That looks like fungus. Here's some information from the Zeiss website regarding fungus. If it is fungus you may be able to halt its progress with an ultraviolet light source (removing the lens and placing it in strong sunlight is one thing to try). If it's not affecting your images too badly then live with it as fungus can permanently and irreparably damage anti-reflection coatings - no real repair is possible once that's happened. As long as you avoid scenes with very high contrast and use a lens hood you should still get usable images.

It's worth keeping some silica packs with your camera gear if you intend to put it away for a period of time. These act as a dehumidifying agent and help to prevent fungus starting in the first place.

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  • \$\begingroup\$ Thank you for the reply. Let me try once with the sunlight, if that doesn't help, then as per the Zeiss website, i will have to take it to a service center, as it is interior of the lens. Thanks! \$\endgroup\$
    – user7370
    Mar 15, 2015 at 15:08
  • \$\begingroup\$ I've never had fungus issues before although I've seen it on a couple of old (and very obsolete) lenses. After seeing your example I'm definitely going to freezer-bag and silica-pack any lenses that aren't going to get used for a while... \$\endgroup\$
    – Darkhausen
    Mar 15, 2015 at 18:24
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    \$\begingroup\$ I think the advice about removing with UV is bad and dangerous. First, if you're going to leave your lens in strong sunlight, be very careful that it isn't focusing that sunlight onto anything! Second, optical glass isn't particularly transparent to a lot of UV wavelengths: if the fungus is on the inside of the lens, it's probably not going to see a whole lot of UV. Third, fungus seems to live quite happily in the open air in sunlight: are you sure UV even has any significant effect? Fourth, even if it kills the fungus, you still have a lens covered in dead fungus. \$\endgroup\$
    – David Richerby
    Mar 15, 2015 at 18:34
  • \$\begingroup\$ David, have you read the info on the Zeiss page? They mention irradiation with UV as a preventative measure. If it was "bad and dangerous" I doubt they would advocate such measures. That covers your second and third points. Fourth point, yes you still have fungus in your lens, but surely it's better to attempt to stop it in its tracks than to simply leave it to continue spreading. Repairs are costly and some people will be happy to make the best of a bad situation. \$\endgroup\$
    – Darkhausen
    Mar 15, 2015 at 18:56
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    \$\begingroup\$ @Darkhausen I read the Zeiss page but missed the part where they say "Short solar radiation or irradiation with UV light may also help avoiding fungus." However, it only says "may help avoid", not "will help avoid" or even "may help cure", so I stand by my comment. \$\endgroup\$
    – David Richerby
    Mar 15, 2015 at 21:53

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