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01-30-2009, 12:55 PM   #16
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Well, that doesn't look too bad, does it? For a broken lens.....

01-30-2009, 02:16 PM   #17
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QuoteOriginally posted by Just1MoreDave Quote
It should be possible. Some solvent to get the elements apart, cleaning, then recementing. How hard can it be? Of course the cement needs to be something that won't alter the optical properties of the lens. The lens coatings need to be intact. It needs to end up with no air bubbles and be a consistent thickness. The lenses need to maintain their alignment. It's like a lot of lens repairs - either you pay someone a lot of money or wing it yourself.
Well, it all is very easy...

First you have to disassemble the lens, take out the cemented elements and bake them in the oven. At around 200 deg Celsius, the glue will get soft and you can separate the elements completely. BEFORE doing that, you will have marked the rotational position of the two lenses with some pencil.

Then you have to clean the residue of the old glue - acetone might work.

Then you need some optical glue. In the long and distant past, opticians used Canada Balm for that. It works - but it takes weeks to cure and I would never let that stuff come near me. It sticks everywhere and is very hard to remove. What you need is modern optical glue, that is cured with UV light. (Edmund Optics is the place to go Edmund Optics)

First build a small rig, that allows complete symmetrical assembly of the two elements. Put only one or two drops of the glue on the lower element and put the second element on top (you have made notes or photographs during disassembly, so you know which side of which lens faces the other, ofcourse), whiggling it carefully around to squeeze all the air bubbles out. There might be some left between the elements and you start all over or you simply ignore some small bubbles (but you won't ignore any dust!).

Let the whole assembly settle for a couple of hours and see, whether the remaining air bubbles get out. Also be sure, that the two lenses are rotated correctly and the pencil marks match!

If everything looks nice, get your UV light and cure the glue.

If that is done, you are nearly done and you can reassemble the whole lens.

--

Now you know, why commercially this is only a repair that will be done to really expensive lenses. Even a repair shop will have to go through the whole effort and that takes much time. I personally would not make such a repair for such a cheap (comparatively) lens - perhaps for my 600mm Apo Schneider-Kreuznach, but not for the old M pancake...

Ben

Last edited by Ben_Edict; 01-31-2009 at 07:29 AM.
01-31-2009, 07:11 AM   #18
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Ben, an incredible post on fixing a lens with seperated elements. I have one of the Sigma 400 apo lenses that needs this done. About 50% had the problem. Friend has same bad lens. We might give this a try. Wow!
thanks
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01-31-2009, 07:33 AM   #19
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QuoteOriginally posted by barondla Quote
Ben, an incredible post on fixing a lens with seperated elements. I have one of the Sigma 400 apo lenses that needs this done. About 50% had the problem. Friend has same bad lens. We might give this a try. Wow!
I guess for the 400 Apo it really is worth a try. Is it the front lement? These should be "quite" easy to fix, as they should have only a moderate curvatore and shouldn't be to sensible to a slight(!) misalignement. On the other hand, the front element is big and you will have more trouble getting out the tiny air bubbles. Use a bit more of the glue, so the the top lens "swims" on the lower one and give it more time to settle. Time for settling is uncritical, as long as you don't have UV light on the assembly (so make that away from any sunny window). If you have a few tiny bubbles left, forget it, it won't ruin contrast.


EDIT: With the big lenses, be extra careful, when heating the lens assembly in the oven. Make it a slow process and control then and now, whether you can already move the lenses against each other. Such bigger glass might crack more easily during heating. Try to separate the elements in a warm environment and let it cool afterwards very slowly, to reduce tensions in the glass. These tensions might otherwise crack the glass at some later point.
Ben

01-31-2009, 07:43 AM   #20
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Ben: Could you please give a simple description of the 'rig" you mention?
To lower one lens on to the other could you use a small suction cup and lower the top lens in correct alignment on to an edge of the lower and slowly angle it down until all the air is moved out.
What do you use to apply the cement and how viscose is it? Liquid like glycerine or thick and ropey like an epoxy?
Thanks in advance for your input.
01-31-2009, 10:43 AM   #21
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QuoteOriginally posted by Ben_Edict Quote
...Now you know, why commercially this is only a repair that will be done to really expensive lenses. Even a repair shop will have to go through the whole effort and that takes much time. I personally would not make such a repair for such a cheap (comparatively) lens - perhaps for my 600mm Apo Schneider-Kreuznach, but not for the old M pancake...
The old M pancake might be good practice, though. I don't think I would want to use a really expensive lens as my first victim.
01-31-2009, 12:43 PM   #22
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QuoteOriginally posted by Arjay Bee Quote
Ben: Could you please give a simple description of the 'rig" you mention?
To lower one lens on to the other could you use a small suction cup and lower the top lens in correct alignment on to an edge of the lower and slowly angle it down until all the air is moved out.
What do you use to apply the cement and how viscose is it? Liquid like glycerine or thick and ropey like an epoxy?
Thanks in advance for your input.
Arjay, the glue is more like glycerine.The "rig" I meant is simply a board (a formica covered particle board or something similar smooth and easy to clean) on which you would make a couple of holes and then you can apply some wood pegs or nails or whatever, that make sure, that the upper lens will stay in place and not float off to one side. If you use three pegs adjusted evenly around the lenses, that should work nicely.

You would apply only one or two or three drops (dependent on the lens size) to the center of the lower lens, lying on the rig. Then lower the upper lens slowly. You don'
t need a suction cup for that, lint free gloves (latex or silk or whatever) will do fine. Just place the second lens centered on the lower lens, the pegs around the lenses will help with this. The weight of the lens alone should spread out the glue evenly. If not, you can take both lenses out of the rig and whiggle them a bit around to distribute the glue. But that should only be a final thing to get the glue completely to the rim. If there are larger glue-less gaps, take the lenses apart and apply a bit more glue for the next trial. This is uncritical, as the glue won't cure without UV light. So (except for the expense and time), there is no problem to take the two lenses apart, when the glue did not distribute evenly and give it a new try.

One thing I forgot to mention: Please clean the two facing lens sides carefully before applying the glue. In my own experience Eclipse with Pec pads is the best solution for getting lenses really clean.

Ben
01-31-2009, 05:49 PM   #23
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Thankyou for the extra detail - I now can visualise what you are doing exactly.

09-16-2009, 11:42 AM   #24
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Did anyone get round to trying any of this? I have 2 lenses in which the elements have started separating. One is a Soligor 500mm f/8.8 mirror lens ( two lens groups separating, I think someone has used this as a buring glass) and a Zeiss S plannar T* 60mm macro (separartion of a doublet in the back of the lens). I got both of these very cheep and had intended to use the mirror lens as a trial before doing anything to the macro. I have access to a lot of specialist equipment but a bit of added confidence would be good before I start cooking my lenses... I have got some quite good photos from the Zeiss but the contrast is a bit poor and would be improved greatly if the separation was fixed. Any sucess stories would be greatly appreciated and would increasse confidence.
09-16-2009, 12:27 PM   #25
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The relative expense of the recommended adhesives ($25 for 1.5 oz of a very low shelf-life item you'll use 3-6 drops of) has kept me from experimenting on the low value lenses I have that need re-cementing.

Lots of info by googling "optic adhesives".

H2
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