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I purchased a used 85-300mm lens online a few days ago. It works fine, with no problems at all (I think), however, I noticed this morning that there is something inside the lens, right between the number 7 and the MC.

Telephoto Lens

I'm not sure if it's fungus, it looks to be some kind of moisture, and it doesn't show up when I look through the viewfinder. However, I'm concerned that this will eventually affect the lens' performance, and eventually spread until the lens is unusable. I really don't want to return it, is there anything I can do about this?

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  • \$\begingroup\$ Have you tried cleaning the front element? Does it come off? \$\endgroup\$
    – NoahL
    Commented Apr 11, 2017 at 16:04
  • \$\begingroup\$ It looks like sort of (liquid) film with different thickness close to 400nm. It may be oil or something like that. Try to gently clean it, it shall go off with your fingerprint too :) \$\endgroup\$
    – Crowley
    Commented Apr 11, 2017 at 16:52
  • \$\begingroup\$ Sorry, something I forgot to mention is that it is inside the glass. It's under the front part so I can't get to it. \$\endgroup\$
    – ToastHouse
    Commented Apr 11, 2017 at 17:04

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The camera lens is comprised on a mix of positive (converging) and negative (diverging) lens elements. This is necessary because as the light rays travel through glass, each color takes on a slightly different path. If only one lens element is used, each color will have a slightly different focal length. The countermeasure is to combine a too strong positive with a weak negative. Both have opposite color errors (chromatic aberration). Additionally other lens elements with different powers made with different glass densities mitigate other lens errors (aberrations). Some of these glass lens elements are spaced with air between them. Others are in direct contact, cemented together with super transparent glue. It was industry standard to use glue (resin) made from the Canadian Balsam pine tree. Over time, this resin can become brittle. Now the cemented lenses can separate if subjected to a blow or heat or cold that induces expansion and contraction. You are seeing a separation of the cement. Sorry, the repair cost is not worth the investment. Plus, finding someone willing to do the work will be near impossible. The good news is, the image degrading from this separation is likely not noticeable.

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Well, it looks like it's interference fringes, so my guess would be that the front element is separating from another element (sometimes two lens elements are cemented together- if the lens is old, the elements may begin to separate). This is seen sometimes in older large format lenses. The 'rainbow effect' is one manifestation but sometimes it's just a shiny spot or spots. It will probably get worse over time.

enter image description here

You can see more examples here. Your case is pretty minor, and any effects would be minimal.

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