Strange pattern in Angulon 90 6.8 front element - balsam separation?

[kaif]

I just received a Schneider Angulon 90 6.8, with a rather strong pattern in it's front element (like a number of c-shaped rainbow arches - see images.) My guess is it's balsam separation? Any thoughts would be much appreciated, especially if you think I'm wrong and it's something different!

[ruilourosa]

Yes separation... a tough lens to glue given the diferential in lens elemsnts radius

Cheers!!!

Rui Lourosa

[Nodda Duma]

Lens separation. It's not difficult to repair with access to proper tools.

[EdSawyer]

True, but these are cheap enough it's probably not worth the hassle, just get another one.

[domaz]

Angulons seem particularly prone to separation, if Ebay is any indication. Bid/buy carefully if you can't inspect the lens in person.

[Nodda Duma]

That means there's a thermal expansion coefficient mismatch between the two elements of the doublet used in the design. What happens is that two CTE-mismatched elements expand and contract over temperature. This induces stress in the bond joint. If the CTE mismatch is slight (my design rule of thumb is 2 ppm/C or less), then the bond can accommodate the stresses without failure. If the mismatch is higher with subsequently higher stresses, then the bond will fail as seen here after years of experiencing normal temperature variations.

It can also happen because of poor surface prep, but if the issue is systemic with this particular model then that points to a design issue.

It rarely happens systemically now because of the wide selection of glasstypes available today, but in the past choices of glass with all the desirable properties were more limited.

[Corran]

My 90mm Angulon did this as well, after I bought it. My guess is it came to the hot, humid south and that caused it.

It flares really badly when oblique light falls on the front element. Otherwise there's no issue.

[Steven Tribe]

These are partial Newton's Rings, which shows there is a tiny air gap in the effected area. The physical optical cause is interference between reflected light wavelengths. Nice indication of the different wavelengths of visual light. You can actually work out the width of the air gap, using the wavelength of light.

This is almost always a problem with hardening synthetic glues - balsam retains a lot of flexibility, allowing the surfaces to slide back and forth.

[John Layton]

A slight segue (with apologies)....but a small number of the lenses I've owned over the years (typically "2nd to 3rd Tier" lenses) have exhibited newton's rings only under fluorescent light - nothing under tungsten or natural light. Any idea why this might be?

[Nodda Duma]

Actually, synthetic adhesives are superior to Canada balsam in almost every way... particularly because tailorability of the chemical formula has provided for a very wide selection of adhesives. If something you need isn't available, companies like Dymax will custom make an optical adhesive.

If Canada balsam was superior, it would still be widely used in the optical industry today. It isn't.

Use it when repairing doublets that were originally bonded with Canada balsam, if you're DIY'ing and have no UV curing capability.

[Steven Tribe]

Actually, synthetic adhesives are superior to Canada balsam in almost every way...
.

Except -well, for earlier synthetic glues, at least - preventing sudden loss of adhesion, through thermal stress.

[Nodda Duma]

Not even then.

Norland 61 was developed in the 1960s specifically for NASA because Canada balsam could not survive the rigors of space flight (think temperature cycling). Aside from a wider range of operational and storage temperatures -- and UV curing -- it also remains pliable long after Canada balsam hardens (lower outgassing). NOA 61 is still in wide use today... it is considered a "good run of the mill" adhesive for doublet bonding, and chances are most of the lens doublets in any of your modern camera lenses are bonded with NOA 61 or something very similar.

Failure correlation isn't to adhesives. Failure correlation is to large CTE mismatch of the elements and exposure temperatures beyond design intent. By that I mean they would fail regardless of the adhesive. It wasn't as easy to match CTE's back then due to more limited selection of glass types -- especially in the post-war period with introduction of higher index / lower dispersion glasses that designers wanted to use. If the designer wasn't careful with controlling the bonding surface radius of curvature, then failures were more likely as the years pass.

Interestingly enough, I've participated in analysis and testing of doublet debonding at temperature, and we found that the bond joint most often fails at cold rather than hot temperatures. The reason is that optical adhesives soften (become more pliable) at hotter temperatures, and become harder (more brittle) at cold temps. The adhesive can thus survive stresses at hot extremes that it can't survive at cold extremes. Thermal shock (significant, rapid change in temperature) is also a cause.

[Steven Tribe]

Interesting.
The cooling problem could explain why I have seen a number of failures in connection with air transport and associated winter storage in Warehouses etc.

[Lachlan 717]

I've found several of the (inner-ring marked) Fujinon lenses get this effect, particularly on edges.

FWIW, I haven't noticed any image degradation from it, though.

[kaif]

Thank you everyone for their detailed advise and information!

My initial instinct would be to just return it, since at the price these go for, it hardly seems worth trying to repair it. Part of me would be curious just to see how much impact this has on the image quality, and with a very rough test putting it in front of the sensor of a Sony A7s I didn't notice any obvious degradation even with some shifting towards the edges of the image cone - but then, I don't have an undamaged lens of the same type here, to compare it to.