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07-11-2010, 04:55 PM   #1
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M 50mm f1.7 circular etching: what is it?


A while ago I got a M 50mm f1.7, claimed to be in excellent condition with no glass flaws other than internal dust. It looked fine and worked fine when I got it. However more recently I was looking through the lens, and I noticed a thin stripe of rough glass near the edge of the rear internal element. It is approximately 2mm constant width, and covers about 120 deg or so of the perimeter. It is out far enough on the edges that it's only visible at f/1.7.

Recently I purchased a second M50/1.7 (also from ebay, also claimed to be clean glass), and this one had the exact same thing in the same location, except this stripe is 360 deg around the perimeter. Some photos:

Photos were taken at an angle from the front, with maglight shining through the rear (most of the crud is 100% crop high ISO noise and dust reflections). The photos don't show the detail of the etching too closely, but they look almost like cross-hatching (i.e. composed of multiple clusters of parallel lines).

Question is, is this residual fungus etching? Or is it some kind of mechanical abrasion damage? The odd thing to me is how regular the shape of the etching is. I'd think biological organisms would have spread from some kind of central point, and the damage wouldn't be so regular. Then again, the inner rear element is a big dome, and there's a ring-shaped gap between it and the outer rear element. Perhaps that gap allowed fungus to grow in such a pattern?

I have no idea. Has anyone else noticed something similar?

07-11-2010, 05:12 PM   #2
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Looks to me like the beginning of element separation. Not sure though, I've only seen it once.
07-12-2010, 02:21 AM   #3
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Agree, it looks like lens separation.

One of my favourite lenses (K 100/4 Macro) has it as well, and it seems to have worsened in the last few months...

Anyone know if the problem gets worse over time and what causes it? is it vibration? or bumps/knocks?
08-10-2010, 10:53 AM   #4
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08-10-2010, 03:23 PM   #5
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QuoteOriginally posted by akerue Quote
Agree, it looks like lens separation.
Anyone know if the problem gets worse over time and what causes it? is it vibration? or bumps/knocks?
From what I've read, lens separation results from cement drying out. Lens elements are cemented together; certain cements dry as they age, and no longer function as glue, and EEK!! the elements separate. Shock shouldn't worsen the condition, but probably won't help either. Can it be cured? Sure, all you need is money. Specialists do such repair work, but if the lens isn't worth kilobucks, don't bother.

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