[Boris_Akunin]

Hi,

while taking photos of a couple of lenses I'm planing to sell, I noticed a few dots that seem to be on the backside of the front element of my Pentax F 70-210/4-5.6.
They are only visible from certain angles and I've never noticed them before. They haven't bothered me yet but now that I'm selling it I need to know what to tell prospective buyers.

I unscrewed the front lens group, the dots seem to be somewhere between the first three elements but those are screwed together very tightly.


Can anyone tell me what this stuff is?

regards
Jan

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[VisualDarkness]

Looks like some kind of separation to me.

[VisualDarkness]

My Sigma 70-200 HSM II looks exactly like this copy: Lens bubbles or fungus? (Sigma 70-200) - Photo.net Pentax Forum

[Boris_Akunin]

I found another example: Bought a dud F 70-210 on eBay: Pentax SLR Talk Forum: Digital Photography Review

I guess I'll try to provoke flare and take some shots to see how much of an impact it has...


EDIT:
I've taken a couple of shots into my desk lamp...
70mm f/4
70mm f/16
70mm f/32
135mm f/4.5
135mm f/32

Those spots at f/32 are probably connected to those on the lens but I don't know what else is...
How does the F70-210 normally behave?

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Last edited by Boris_Akunin; 06-05-2013 at 04:54 PM.

[Odinz]

Unfortunately that looks like lens separation. I had several older lenses, mostly third party lenses circa the late 70's to early 80's that look exactly like that. Or should I say 'Did' look exactly like that. When they were in that stage the effect on photos was minimal, but sad to say most of them progressed over the years to the stage where the lens became unusable. I have found that having the separation on the front group of lenses has the least effect on the optical quality, when it appears in the middle or the rear elements it's a lot worse. It supposedly can be repaired, but the cost and effort would be prohibitive for all but the most valuable lenses.

[stevebrot]

I have seen similar spots on Schneider lenses for large format. Prevailing opinion among the large format crowd is that they don't hurt anything. That being said, it may well be an issue with smaller formats.


Steve

[Boris_Akunin]

Thanks guys,
I guess I'll document the effects as far as they exist and sell it with full disclosure.
It is after all still perfectly usable...

[VisualDarkness]

I have seen similar spots on Schneider lenses for large format. Prevailing opinion among the large format crowd is that they don't hurt anything. That being said, it may well be an issue with smaller formats.


Steve

The Schneider ones are between the barrel and the element, not between elements like this. Search for Schneideritis (poor them even as they even get to be a word for it).

[stevebrot]

The Schneider ones are between the barrel and the element, not between elements like this. Search for Schneideritis (poor them even as they even get to be a word for it).

I am familiar with what you are talking about (paint separation from masked portions of the glass), though has been my understanding is that there is more to the issue. The examples I have seen (in person) have spots that extend a fair distance from the edge, though to be fair, it is possible that refraction could make the spots appear to be where they aren't.


Steve