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Repairing cemented lenses
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PostPosted: Tue Dec 30, 2008 7:25 pm    Post subject: Repairing cemented lenses Reply with quote

Prompted by a question from member hk300, I thought that this description of my repair of a cemented lens element may be interesting, if not helpful!

When I repaired the Tokina 80-200 f2.8 MF zoom, the problem was the front element of the second group, which is a cemented doublet. The fault was failure of the cement causing severe loss of contrast. I reached it by removing the elements of the front group, and then extracting the doublet.

To separate the elements, you need to soak the lens for 24-48 hours in a suitable solvent. I tried acetone first, but that made no difference. I then used methylene chloride (used to be in paint stripper, but that is now quite dilute, so I bought a bottle of ProWeld which is used as polystyrene cement for building plastic models, higher concentration). After 24 hours it looked as if this had not worked, but the elements of the lens were just clinging together because of the very close curve fit, and just needed to be slid apart. If the ProWeld had not worked, my next attempt would have been with Methyl Ethyl Ketone (MEK). Needless to say all these chemicals are quite toxic, and demand good ventilation!

Once the cement is dissolved, the residue will clean off very easily with acetone, ready for re-cementing.

To re-assemble the lens I used Norland NOA61 uv-curing adhesive from Edmunds Optical. A few drops are placed in the "cup" of the concave lens, and then the two elements are "wrung" together which squeezes out the excess cement. I used a V-block to keep the elements centered since they are the same diameter, if yours are different diameters you will need to make a little jig to centre them.

To cure the adhesive I just exposed it to British summer sun-light on a window sill in my house - for about 20 seconds intially, which makes the joint strong enough to handle so that you can use acetone to wipe off excess adhesive - then another 10 minutes for complete cure. I then left it for 3 days to ensure no solvents or vapours were left before I re-assembled the elements in to the zoom lens.

As far as I can tell, the result is 100% successful, with normal contrast and sharpness evident in images taken with the lens.

Have fun!

Angus


PostPosted: Tue Dec 30, 2008 7:59 pm    Post subject: Reply with quote

Many,many thanks! This is a very important information!


PostPosted: Tue Dec 30, 2008 8:00 pm    Post subject: Reply with quote

This forum is truly becoming a source of great DIY and general construction ideas... Smile I'm happy the first looks got me interested in this place, there are so many bogus and name-calling sites around that only wastes your time...

Not so here Smile Thank you for the tip. I'm not too sure that the solvent that worked for this cement will work on every lens, but if everything else fails, there's always MEK and later Toluol or Xylene... Yummy! Makes me miss my first years in the graphics business as a rotogravure print operator...


PostPosted: Tue Dec 30, 2008 8:42 pm    Post subject: Re: Repairing cemented lenses Reply with quote

Photomac wrote:
...Once the cement is dissolved, the residue will clean off very easily with acetone, ready for re-cementing.

...

As far as I can tell, the result is 100% successful, with normal contrast and sharpness evident in images taken with the lens.

...


Wow! Amazing! I would have risked a bet on that you are not able to re-alling these lenses properly again! Shocked


Last edited by LucisPictor on Wed Dec 31, 2008 9:57 am; edited 1 time in total


PostPosted: Tue Dec 30, 2008 9:09 pm    Post subject: Reply with quote

Thanks for the comments everyone.

Suede you are right, the solvent will depend on the adhesive used originally. I suspect that the Tokina was still using Canada Balsam - the separation had that yellow appearance. Still if you get into MEK or Xylene you can save good money on the AquaVit, Whisky, Schnapps or whatever means you normally use of inducing unconsciousness Laughing

Lucis Pictor - the alignment was easier since both elements were the same diameter, but you properly identify the critical thing - having the elements centred with respect to one another, and with no 'wedge' or tilt. May I also say that your posts, images and information have been a great help in my re-discovering photography.

Regards

Angus


PostPosted: Tue Dec 30, 2008 10:00 pm    Post subject: Reply with quote

Angus, thanks very much for the useful advice. Could I ask you a couple of questions? :

How do you recognise that the old cement has broken down? Is there a visible haze or cloudiness, or is the problem only apparent in the photographs?

and

where can you obtain the various solvents?


PostPosted: Wed Dec 31, 2008 10:30 am    Post subject: Reply with quote

Peter,
I bought the Tokina 80-200 2.8 for about 1/10 of the normal selling price because it had the fault, and I wanted to try the process that I described. So from the start, this lens had a cloudy film obscuring the view through the lens, and which was clearly visible as being within the double element.

The effect on images was as if most of the field of view was being obscured by wood-smoke from a pile of burning leaves/branches! I would guess that the problem would start to appear in images first, as if an element needed cleaning, and it would become clear what the problem was when cleaning fails.

As for supplies of solvents, the most fruitful area I have found is online, from shops specialising in modelling - plastic kits, railways, radio-controlled aircraft. As long as you know the name of the chemical you want, search for it and you will usually get entries from manufacturers/suppliers web-sites - they have to list them given the nature of these solvents.

Regards

Angus


PostPosted: Wed Dec 31, 2008 1:00 pm    Post subject: Reply with quote

I know, and it's really a sad story... When I got technical responsibility of the gravure print department I had to vouch for three of the older employees going into early retirement (that they weren't alcoholics), and supply the government departments with medical papers confirming my statement. Their (premature) pensions were on the brink of getting revoked on reasons of suspicions of grave alcoholism.

Long exposures to these kind of solvents cause (among other things) liver and brain damage virtually impossible to discern from the ones you get from heavy alcohol abuse. We had to fight one of the cases all the way into official court, social services and social insurance refused to acknowledge that he was "clean" from an alcohol point of view. It took 3 months of blood testing two times a week (all totally clean) to finally convince them that the person wasn't an alcoholic. After two years, he finally got redemption and a ~50k settlement "bonus". During these two years we continued his employment salary even if he wasn't actually working, we were the ones offering him early retirement from the start - so we were in part responsible for his situation... :/ We got the money refunded by the government in the end. All's well that ends well they say Smile


PostPosted: Wed Dec 31, 2008 4:52 pm    Post subject: Reply with quote

Great post Angus ... glad you shared this information!


PostPosted: Thu Jan 01, 2009 12:58 am    Post subject: Reply with quote

@Photomac
Superbly useful info, thanks for that.

@Suede
A friend of mine was exposed to some cleaning solvent in the course of his mechanical fitting work and his employer refused point blank to take any responsibility for it. Just one of many thousands of similar tales, I'm sure, but knowing the bloke directly, the shock I had when I saw him afterwards was more pertinent.
His entire head and facial features had been affected, so much so I didn't recognise him at first - apparently the damage was much worse during the time I hadn't seen him - it had subsided a fair bit over the following months.
Anyway, the employer's insurance company are refusing to pay out on the basis of his health is fine and he can't prove it was the solvent they supplied that was at fault. Also, he can't prove his loss of looks is directly caused by that. His facial features are all affected - skin coarsened, etc.


When I think back to the number of times and the amount of solvents I've been exposed to, I count myself dead lucky to still be in one piece today.


PostPosted: Thu Jan 01, 2009 10:23 am    Post subject: Reply with quote

Farside wrote:
the employer's insurance company are refusing to pay out on the basis of his health is fine and he can't prove it was the solvent they supplied that was at fault. Also, he can't prove his loss of looks is directly caused by that. His facial features are all affected - skin coarsened, etc.

B'stards!


PostPosted: Wed Feb 25, 2009 7:07 am    Post subject: Reply with quote

Interesting sharing.

Quick to ID the solvent, does anyone know what glue thus solvent for the Kiron 28-85mm stovepipe, the last pair is very hazy, split for cleaning?

The 2 lens reflections, one purplish reflection, the other is slightly yellowish, of the flo tube if that helps?

thanks.


PostPosted: Wed Feb 25, 2009 12:04 pm    Post subject: Reply with quote

peterqd wrote:
Angus, thanks very much for the useful advice. Could I ask you a couple of questions? :

How do you recognise that the old cement has broken down? Is there a visible haze or cloudiness, or is the problem only apparent in the photographs?



You can see it looking at the surface of the lens too.

In Argentine we called "rainbow" to him. The refleccin of the lens (front or rear)is as if you saw oil expanded on the pavement of the street. Of several colors and metalized appearance. Hum...it's another case of pegament problem.

Rino.