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Blackbeard_ben
Tinkerer
Username: Blackbeard_ben
Post Number: 3
Registered: 06-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, February 14, 2010 - 06:21 pm: | 
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I sold a Nikon 20mm f/2.8 lens in pretty rough condition (front element scratched up, but rear element perfect AFAICT), and the buyer in Thailand is claiming that there is a hole in the coating on the rear element, and that it's causing the multicoating to peel off.
I've never heard anything like that before, and from my experience with lenses (and coatings being many layers thick), this isn't even possible as far as I know. I know damaging coatings isn't very difficult - but having them peel straight off? I have sever lenses with gashes in the glass and no issues whatsoever.
He says that he took it to a dealer and the tech says the same thing. I don't trust him (despite his good feedback), and I've requested photos of the lens and his repair assessment receipt.
Here's his word on the rear element:
"I’ve seen one crystal spot at rear optic, like
dead spot on LCD. This will make a big problem in
this near future. Cos the lens coated will come
off more. Even the cleaning mark at the center
was the second issue."
Crystal spot like a dead pixel on an LCD? I have no idea what he's talking about, it doesn't make any sense at all. Is this guy trying to scam me straight up, or is this something that can actually happen to lenses? I'll have to see what his photos look like before I make any decisions, of course. I know that it looked perfectly fine to me when I shipped it out, and I have a good photo of the rear element to prove it. |
Glenn
Tinkerer
Username: Glenn
Post Number: 750
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 17, 2010 - 09:02 am: |
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Complete load of crap, you cannot compare damage on optical coatings to dead pixels and coatings do not 'peel' - they are too thin for a start. However if the Tech has found a small area of fungal attack, then his assessment of further damage occurring unless the lens is cleaned is obviously correct.
As you admit the lens was pretty rough, are you absolutely positive that the rear element had a completely sound internal surface? How did you examine the internal surface of the rear element? |
Tom_cheshire
Tinkerer
Username: Tom_cheshire
Post Number: 272
Registered: 04-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 17, 2010 - 06:42 pm: |
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Ok, first, lens coatings are not "many layers thick". Multicoated lenses refers to a single layer of anti-reflective coating on each glass surface (vs only one or two glass elements in a lens being coated as was the norm in the old days). There is not a layer of coating on top of a layer of coating.
Next, if fungus or something like that was eating the coating it is possible to have the coating wipe off when wiped clean. This does not mean the entire coating is going to come off like an omelet out of a pan. It means whatever spot, dot or "branch/vein" of fungus was on the coating has the potential to take the coating off with it when cleaned.
As for "dead pixel" that is not a proper or fair comparison. Light will flow through a lens, coatings or not, damaged coatings or not and will not be noticeable on a negative or final photo.
As for the coating situation getting worse, well, if the fungus is gone there is nothing to keep eating away at the coating. It will not suddenly decide to peel off like an old piece of Scotch tape somewhere in the future.
Ok, what you do is offer a partial refund. That always works and may even be the intention. |
Sevo
Tinkerer
Username: Sevo
Post Number: 59
Registered: 09-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, February 18, 2010 - 02:28 am: |
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As a matter of fact, multicoated is indeed multiple layers of coating on top of each other, to reduce reflection in several spectral ranges rather than one.
Lenses coated on every surface appeared pretty early on, even early forties Zeiss lenses are coated on all relevant surfaces, if they are coated at all - it was not until after WWII that cheap manufacturers came up with lenses which were cosmetically coated on the outer side of the front lens only.
Pre Zeiss process coatings (or rather surface treatments) often were so poorly attached that only inner surfaces were treated - some of these indeed peel off when touched. Misapplied vacuum coatings can come off too - I own one 40's Schneider prototype internally filled with "gold dust", i.e. flaked off coating fragments.
But a Nikon 20/2.8 would be mid eighties or younger (prior to that, they had a long series of 20/4.0 and 3.5 lenses), decades past the age of coating problems (by the eighties, coatings tend to harden rather than soften a lens surface) - and neither do coatings "flake off at a hole".
My guess is that the buyer himself created a cleaning mark or coating damage in a poorly executed cleaning attempt and now tries to haggle for a refund.
Sevo |
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