Would spots of separation on rear lens affect image quality?

[Mike in NY]

This lens is currently on the auction site and is listed for ~half the price of others like it, because it has some small spots of separation on the rear lens. The seller claims this would not affect image quality. I'm curious to know what others think about the potential impact on image quality. Thanks.

[Dan Fromm]

I could be mistaken, but to my inexperienced eye those dendritic thingies look like fungus, not separation. Either way, if I couldn't afford to lose the money I'd pass on that lens.

[rdeloe]

I don't think that's separation... Looks like Mr. Fungus has moved into that lens.

[Mike in NY]

I also wondered if it might be fungus given the snowflake-like shape, but was going with the seller's description. I think you both are right.

[Steven Tribe]

These " organic growth" figures are an uncommon sign of problems in a cement layer - quite a beautiful result!

Something is thrown out of the balsam mix solution which grows like a crystal, snowdrop or fungus mould. Perhaps impurities in the balsam have "seeded" the initial crystalisation.

[Nodda Duma]

I just repaired a lens for a photographer that had similar issue in the cemented layer. It looked just like that and my first thought was fungus, but was definitely a problem with the cement. When the elements were cleaned and stripped of the old cement, no artifacts remained. Rebonded, they looked brand new.

Cheers,
Jason

[Emmanuel BIGLER]

Would spots of separation on rear lens affect image quality?

Coming late to this discussion, as far as I have read, so far I did not see any answer to the original question.

No surprise, this is a frequently asked question here.

If the defect was fungus, actually I would not purchase the lens by fear of contaminating all my lenses at home. Hence I would not even raise the question of image quality delivered by such a damaged lens.
Am I kidding? May, be but as far as I remember, the official web page for Zeiss customer service clearly states that they will not accept for repair any lens contaminated by fungus, period.

Well, now let's assume that the defect is not fungus. I'm not sure that recent LF lenses (say: manufactured after 1970) actually use the old technique of Canada Balsam to cement doublets (or triplets), but this is another issue. The nice thing with Canada balsam is that cementing is reversible. Modern techniques involve so-called "UV-glues" which, to the best of my knowledge, are not as reversible as balsam was, to say the least.

Then, the answer to the question ...
Would spots of separation on rear lens affect image quality?
... is relatively simple ; very probably, NO, as far as the surface of the defect is only a small fraction of the lens element surface, say, a few percent, you will not see anything wrong in your images. Except, may be, that the damaged lens will be slightly more sensitive to stray light.

If I'm the seller, immediately, I would argue: hey, the experts on the US LF forum guarantee that you won't see anything , hence please buy my lens at my price!
And if I am the (potential and cautious) buyer, I say: hey, your lens is defective, may be contaminated by horrible fungus; hence I will never buy it except as a defective item, at a small price only.

So to me the question is only marginally technical, it is mostly a commercial issue, a question of offer and demand

[Dan Fromm]

Emmanuel, as I said, it the OP can afford to buy a lens that is unusable (the worst case, not guaranteed) and the price is very good then buying it may be a good gamble.

The risk from those dendritic thingies is veiling flare more than loss of sharpness.

[Emmanuel BIGLER]

The risk from those dendritic thingies is veiling flare more than loss of sharpness.

Hi Dan! Exactly! At least this is my feeling.

Actually, I just got, once, a lens affected by fungus. I can't speak about that here since the lens was for use with a film format forbidden here.
The lens was for sale on a well-known auction site.
I asked the seller to publicly guarantee that the lens was free of fungus. The seller publicly stated in written on the auction site that the lens was free of fungus.
I eventually won the auction and got the lens, it came in a very nice vintage leather case (should I say that I love vintage lens leather cases?) ... but even before opening the case, I could perceive a distinctive smell of moisture/fungus.
Nice fungus shapes were visible on the rear lens element. A nice academic case for a lecture for students about fungus on vintage glasses
I got a partial refund from the auction site, but could not argue with the seller who by a miracle had vanished from the listings of the auction site immediately after the money was transferred, by a bank transfer (a means of payment I try to avoid now for second-hand items bought on auction sites).

[Mike in NY]

Thank you for the continued discussion; it's really helpful. I'm not sure I'd pay half of the going price (in normal condition) for one of these lenses, but maybe a third. BTW, are we prohibited from saying "eBay" on the forum? I've always wondered that. I just call it by name, and often see that some other members refer to the "auction site" or derivations of its actual name. Just wondering. So far I've never been called out for typing that particular four letter word.