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How can balsam separation between glued elements be fixed?

I'm planning to try to fix a couple lenses because what else am I going to do with them?

It's about looking nice, optimizing image quality, and completeness. One of the lenses (Tamron SP 28-135/4-4.5) had a stuck zoom (with four moving groups), oily elements, and sticky residue, as well as the usual grit and grime that lenses collect after washing ashore on the beach. (Why do so many lenses have sand in them?) All that's left to finish is to fix the balsam separation and calibrate focus.

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    I think the received wisdom is 'you can't', at least not economically. – user82065 Apr 12 '19 at 14:32
  • Just use them: I've used lenses with element separation for years with no ill effect at all. If you want the lens to look pretty (for some value of pretty) then you can try to fix it, if you want a lens to take pictures with, don't bother. – user82065 Apr 12 '19 at 18:03
  • I think modern lenses more likely use optical epoxies rather than balsam. – Eric S Feb 29 '20 at 0:09
  • @EricShain What do you consider modern? I haven't personally encountered any AF-capable lens with separation issues. – xiota Feb 29 '20 at 0:12
  • I’m certainly not an expert, but I used optical epoxies more than 20 years ago. – Eric S Feb 29 '20 at 3:32
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Viable methods of repair depend on the extent of the damage and type of cement involved.

Repairing minor separation (including small discrete bubbles and crazing)

Balsam is used in the preparation of permanent histology slides, which may develop separation and bubbles. A trick I learned from a lab tech is to stick the slides in an oven. The heat melts the balsam, allowing it to self-heal as the bubbles migrate out from under the cover slip.

The same approach can be applied to lenses. Set the oven to just over 300F, and let the lens sit for 15-30 minutes. Make sure the concave surface of the lower element is facing upward so that bubbles will migrate upward, toward the edge of the seam. If bubbles migrate toward the center, just turn the elements over and repeat until all the bubbles have migrated out of the lens.

Repairing major separation

If baking the element for up to an hour doesn't resolve the problem, the elements need to be completely separated and recemented.

  • Separate the elements. Balsam melts at about 300F (149C). So methods discussed on forums, such as boiling in water (212F/100C), will not work. Cooking elements in oil will work, but is unnecessarily messy. Put the element in an oven set to just above 300F (149C) for about 15 minutes. Then carefully slide the elements apart while they're still hot.

    Another option is to use a hot plate (or equivalent). Watch for the balsam to melt.

  • Centering the elements. Most lens elements are designed so that the mechanical center is the optical center. Most lens housings are designed to hold elements so that their optical centers align. So setting the cemented elements in their final resting place will appropriately align them. It's also possible to build a simple jig to hold the elements while they set.

  • Recementing the elements. The difficult part is obtaining a suitable optical cement. Canada balsam does not appear to be available at everyone's favorite auction site, except as essential oils or food supplements.

    I have tried a few different materials, but they are unsuitable for use in photographic lenses. Further attempts are pending my ability to source real balsam or another suitable material.

Cautions

  • Consumer ovens may not be accurate, so you may have to increase the temperature.
  • Wear eye protection and heat-resistant protective gear.

References

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  • Any success? Also, have you considered using UV cement and a curing lamp? Flashlight people routinely cure UV optical cements using cheap UV flashlights (Nichia 365nm LEDs were mentioned). – Christian Chapman Dec 1 '20 at 17:35
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You disassemble the included elements, remove all of the balsom from both surfaces, and then glue them back together. To guarantee proper alignment when re-gluing them, you probably need some fairly advanced optical lab equipment or (for lower cost lenses) an original jig that the manufacturer may have used when the lens was made.

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    This answer is not helpful because it lacks sufficient detail to perform the repair. For instance, how to separate the glued elements. – xiota Feb 26 '20 at 23:25
  • @xiota Insisting on such information ignores the second sentence of the answer. – Michael C Feb 27 '20 at 17:26
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    The second sentence is not helpful either. – xiota Feb 27 '20 at 19:43
  • It's true, and in that sense it is helpful, even if it is not what you wish to hear. Two other users seem to have found the answer helpful. Sorry if that upsets your apple cart. – Michael C Feb 28 '20 at 2:08
  • Then by all means, please write an answer that you consider helpful! – Michael C Feb 28 '20 at 8:29

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