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.
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?
Answer
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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