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I've noticed a old few lenses have some type of black substance (residue, coating, or powder) on some of the elements, especially the inner surfaces between the first and second element. The substance isn't visible (therefore, no picture). I only know it's there after wiping the lens and seeing what appears to be a black powder on the cloth.

The lenses appear to be from the 1970-1980s. The first lens I noticed the problem in was a 135/2.8 prime for Canon FD mount. Most recently, I have noticed it on a Tamron SP 28-135/4-4.5 BBAR MC lens (Adaptall mount).

It occurs to me that this stuff may be on the lens intentionally as an antireflective coating. While I hope not, even if it is, it would still be beneficial to clean away because the lenses do improve in wide-open sharpness after cleaning. Stopped down performance is negligibly affected, and I don't notice any particular change in flaring.

Is this black substance there intentionally? If not, what could be getting into the insides of lenses to coat the elements? 

  • If it's not there intentionally, I do have a guess about what it may be... – xiota Apr 10 at 21:01
  • Does it seem to be worse in zoom lenses? – Peter Taylor Apr 11 at 6:56
  • @PeterTaylor - The first lens I recall noticing the problem was a prime. I've seen it in both primes and zooms. Zooms tend to have more elements, so more opportunity for elements to be affected, but other than that, haven't noticed it being worse in one or the other. – xiota Apr 11 at 7:39
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    Powder wouldn't be used as an antireflective coating on a lens element - it might be antireflective paint that has come off and settled on the lenses, or just plain soot from the environment... Maybe the lens was stored in a way that left it statically charged (which makes everything attract soot)? – rackandboneman Apr 11 at 10:31
  • Soot? Like, from burning coal and candles? – xiota Apr 11 at 10:59
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If this stuff is on the inner surfaces of the lens (so, not the back of the rear element, and only accessible by taking the lens to bits as you must have done) then it is very likely to be decomposed antireflective paint from the inner surface of the lens barrel. Various lens manufacturers used paints which broke up over time: look up 'Schneideritis' for a related symptom. Depending on what the results of the decomposition were, the residue can get dumped onto lens surfaces.

It is also possible that it's decomposed (oxidised) oil,or just general environmental crud which has leaked into the lens during focus movements.

In any case, it's not meant to be there.

  • That wasn't my guess, but seems reasonable. As far as paint is concerned, I'd been assuming the parts were made of anodized aluminum, but a few lenses did seem to have something that seemed like ink coming off of them. Also, the edges of some lens elements are colored black with something that seems similar to the black substance I ask about in my question. – xiota Apr 11 at 11:09
  • Zoom was jammed on the aforementioned Tamron lens. So I disassembled it to attempt to fix it. Meanwhile, I checked whether the elements needed cleaning. – xiota Apr 11 at 11:14
  • Sorry, I wasn't meaning to imply you shouldn't have disassembled them, I just wanted to confirm that it was on internal surfaces. – user82065 Apr 11 at 12:34
  • I didn't read as if you were implying that I shouldn't have disassembled them. Was just giving background on why I was working on that particular lens. Sometimes when a lens just isn't as sharp as I expect it to be, I'll try cleaning the elements because I know old lenses can have an oily residue and this strange black stuff. – xiota Apr 11 at 12:52

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