Anyone know anything about this and what does it look like in a lens which has it? Is it repairable?

It is repairable but expensive, I have a little on an early Sironar, kinda looks like a rainbow little circle at the edge of the front elements. But on older lenses it's quite different. Hard to describe.

Balham Optical (http://www.optil.co.uk/) in the UK. There's a very current similar thread on the LFPF and a link to a US repairer.

Ian

A follow-up. I have seen a website with DIY repairs for older balsam cement fairly recently.

Ian

It's as Ian describes. If it is limited to the edges it's not usually too much of a problem. I have an enlarging lens that is badly affected, but still prints well.

ah, looks like its gonna be expensive if I want to get it fixed. I'll get a quote and we'll see if its worth it.

It looks like a darkish ring going right round the lens about 1/8 the way in from the edge. If I close down 2 stops I can't see it but I would like to use the lense wide open. The ring is perfectly symetrical and the same thickness all the way round.

Camtech Photographic here in Hamilton Canada saved an old Velostigmat that had completely separated. And reasonably too. No Interest in them. But I have been giving them my patronage for more than 25 years.
http://www.camtechphoto.com/
Regards
Bill

I'm in the UK so its either back to zeiss or Optical Instruments (Balham UK)

Rob, what is the lens? I ask because most modern lenses cemented elements are cemented with a uv-curing synthetic, not with Canada balsam. Fixing separated balsam is much easier than fixing the synthetic.

ah, looks like its gonna be expensive if I want to get it fixed. I'll get a quote and we'll see if its worth it.

It looks like a darkish ring going right round the lens about 1/8 the way in from the edge. If I close down 2 stops I can't see it but I would like to use the lense wide open. The ring is perfectly symetrical and the same thickness all the way round.

This doesn't sound like cement failure, normally would be a more irregular pattern. The other descriptions here are typical (rainbow apperance, probably over a crescent shaped area.)
I can confirm that, as another poster stated, use of UV curable adhesive (e.g. Norland 61) is pretty standard now for assembling doublets and that decementing them can be difficult and you run the risk of damaging one of the elements so don't be surprised if a repair shop only takes the job on a best effort basis.
As a general rule you will reduce the chances of inducing this type of failure by avoiding temperature and humidity extremes and especially sudden transitions between extremes.

Rob, what is the lens? I ask because most modern lenses cemented elements are cemented with a uv-curing synthetic, not with Canada balsam. Fixing separated balsam is much easier than fixing the synthetic.

Contax zeiss 28-85 vario

16 elements I think:o

Rob, what is the lens? I ask because most modern lenses cemented elements are cemented with a uv-curing synthetic, not with Canada balsam. Fixing separated balsam is much easier than fixing the synthetic.

In the course of my work I had a TV camera's multi-element colour correction filter de-laminated. The image had an overall cloudy appearance, but the effect when studying the filter was as if the affected plane had a host of worm casts. Removal of the old cement was the hardest part, but the repair was relatively simple. An optical grade of Loctite adhesive was used, held in moderate compression and subjected to about half an hour of the correct wavelength of UV. It was a very successful repair.

Steve

Here are a coupke of links to DIY repair info. They're interesting even if you don't do it yourself.

http://www.pairlist.net/pipermail/largeformat/2005q3/019094.html

http://www.skgrimes.com/popsci/index.htm

I now suspect that the ring I see is more likely what it looks like. i.e. an oily ring and not balsam separation. The lens has been unused for quite a few years and has been packed away in a vertical orientation. I think it may be have been caused by very slow flow of some lubricant from the internal workings of the focus mechanism.
But the only way to be sure is to take it apart.