Repolishing old lenses with fungus, balsam separation

[kfed1984]

Followed your project over from our comment thread on my YouTube channel (Adam Welch). This is quite interesting and as others have said thanks a lot for taking the time to share the process. Glad to see it worked out well.

The most interesting part will be balsam separation fix, I already have a plan, but that will be later. I will share the before/after photos of the lens fungus etch when I get back to polishing the other side of the lens.

www.largeformatphotography.info is a really good blog, for anything large format related, technical stuff. The other good one is photrio

[ic-racer]

Nice, thanks for sharing.

This may not apply to still photography, but I understand some Kern Yvar Cine lenses were only coated on front and rear surfaces. Therefore, the internal surfaces could be polished without worrying about the lens coating. I have not tried it. For some [good] reason all my old Cine lenses have clear elements.

[kfed1984]

Nice, thanks for sharing.

This may not apply to still photography, but I understand some Kern Yvar Cine lenses were only coated on front and rear surfaces. Therefore, the internal surfaces could be polished without worrying about the lens coating. I have not tried it. For some [good] reason all my old Cine lenses have clear elements.

Usually it is one or two surfaces that are damaged by fungus, and in a typical 4 element lens there are 8 surfaces. So we will loose about 1/4 of the total anti-reflection quality if the coatings are polished off.

[Mark J]

This is all good and makes sense. I am familiar with the process, I got into optics by grinding and polishing telescope mirrors. the pitch needs to be optical quality.
Yes, modern AR coatings are dense and very hard. they will take ages to polish off.

[kfed1984]

This is all good and makes sense. I am familiar with the process, I got into optics by grinding and polishing telescope mirrors. the pitch needs to be optical quality.
Yes, modern AR coatings are dense and very hard. they will take ages to polish off.

Thank you for the feedback. I came across a passage in "How to Make a Telescope" that surface accuracy on lenses needs to be 1/4 of what is required in mirrors. Not sure if this makes any potential refiguring/despherizing done by my polishing technique less damaging than it would be on mirrors, as I am not doing any Foucault testing on the lenses. Just trusting that my polishing process did not alter the sphere too much.

Do you have any experience with cementing of lenses and balsam separation or are you only into mirror making?

I bought the pitch and cerium oxide from here: https://firsthanddiscovery.com/gugolz-73-pitch.html

[kfed1984]

"How to Make a Telescope" by Jean Texereau. This book has good illustrations and is considered a classic for this topic. Archive does not have a downloadable version, but you can borrow the book virtually for one hour.
https://archive.org/details/howtomak...0texe/mode/2up

I have the paper version.


Amateur Telescope Making, book 3. Lots of good stuff on lens making: https://archive.org/details/amateurt...p?view=theater

[Mark J]

The surface accuracy on photo lenses is typically less stringent than on telescope optics - eg. 1 to 3 fringes, rather than less than 0.5 fringes on a telescope.

I work in an optical company. Balsam went out decades ago, for us. We use either UV-curing cements or specific optical epoxies.

[kfed1984]

The surface accuracy on photo lenses is typically less stringent than on telescope optics - eg. 1 to 3 fringes, rather than less than 0.5 fringes on a telescope.
I work in an optical company. Balsam went out decades ago, for us. We use either UV-curing cements or specific optical epoxies.

Concerning realignment of cemented optics, I am thinking of using a cast-plaster jig, molded around the lenses before separating them, for radial realignment. Prior to that, coating them with Vaseline for easier release and with modeling clay applied in strategic places to keep the plaster out. And, marking the lenses on the edges for angular re-alignment of lenses to each other and to the plaster jig.

Then using heat to separate lenses, solvent cleaning, and recementing with UV-curable resin in the plaster jig. Balsam I heard takes a long time to set, so UV-resin it is.

[Steven Tribe]

Later on I want to add information on balsam separation repair, which will involve creating an alignment fixture, separating the lenses with heat, cleaning old balsam, applying new lens cement, and realigning/cementing back to original position.

There are plenty of threads here which cover balsam renewal.
“….separating the lens with heat….” Doesn’t sound like a suitable method with irreplaceable Antique lenses!
Originally, pitch was only used to fix the glass to the “lathe” - a more stable material was used to make the optical surface polishing tool.

Amateur cementing is best done with Canada balsam.
Because:
- easy to redo. Yes, it may be necessary!
- much cheaper.
- synthetics have short shelf lives. Viscosity of Balsam is easily adjusted.
- balsam is non-toxic.

[Mark J]

Originally, pitch was only used to fix the glass to the “lathe” - a more stable material was used to make the optical surface polishing tool.
.

Well, the polishing tool has a former made out of metal, but the polishing surface was a layer of pitch that conforms to the lenses and holds the rouge or cerium. Polishing with pitch was industry-standard for most (quality) optics until about 25 years ago, from where it has gradually been phased out by polyurethane and then other polymers used in the high-speed automated machines. However we still retain pitch polishing in out factory for small batches and certain special materials.