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What is happening with this lens? Bought in a box of used lenses. Clearly it has met with some abuse, but I'm not familiar with what's actually happening. It has this colorful 'C' shape somewhere inside of it.

Lens with strange blemish.

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Hard to tell what it is without examining the lens in person. Could be something benign, like lens coatings, or something impossible to fix, like balsam separation. Based on the age, type, and manufacturer of the lens, I suspect it could be oil on a lens element. Supporting evidence would include a "greasy" appearance and "sticky" aperture. The problem can be fixed with a CLA (clean, lube, adjust).

If the lens works fine as-is (probably not), I'd consider leaving it alone if the affected element is deep within the lens. It's an old zoom with clear damage that may not be worth the cost to fix. If you try to clean it yourself, you risk being unable to reassemble it because old zoom lenses can be difficult to reassemble. Disassembly of lenses made by Kino Precision is complicated by parts that are unnecessarily glued together.

Here's a picture of an oily residue behind the front element of a Kiron 28-210/4-5.6 Macro MC prior to cleaning:

oily residue

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  • At least for some kino precision made lenses, pdf service manuals can be found on the net. However, these same manuals tell you to have at the glue(!) securing some of the parts together with acetone(!). Wonder what hit that lens though - once dropped a KP vivitar lens on a hard floor from a foot or two and there is no trace of that happening left :)
    – rackandboneman
    Jan 29, 2019 at 8:48
  • @rackandboneman It's the unnecessary glue that makes disassembling them such a pain. Reassembling is annoying simply because zoom lenses have so many parts to fit back together.
    – xiota
    Jan 29, 2019 at 8:51
  • @rackandboneman I don't think anything necessarily "hit" the lens. Based on what I've read, lubricants in old Kino Precision lenses were susceptible to outgassing, which can coat lens elements and cause sticky apertures.
    – xiota
    Jan 29, 2019 at 8:55
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    Maybe consumer canons get bent filter threads from the gas pressure... unlikely to happen with KP :)
    – rackandboneman
    Jan 29, 2019 at 9:20
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    @rackandboneman Excess gas pressure is more of a destructive problem for cannons than canons.
    – Michael C
    Jan 30, 2019 at 22:17

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