Repolishing old lenses with fungus, balsam separation

[Mark J]

Unfortunately no, not that I'm aware of. That's why optical shops use either a test plate ( sphere ) matched to the design radius, or nowadays, interferometers stationed in the shop.
You will just have to rely on getting the lap running smoothly with the right kind of steady drag , plus a moderate ( 1/3 diameter) stroke in various directions. This should get you a surface that's within a couple of fringes, which is all that's required for a photo lens - less stringent than a telescope mirror.

[Nodda Duma]

Stumbled across this thread. Interesting reading. Some good info, some myths, some not so good info.

Mark J: Your quetion about how lenses wre aligned long ago.

1) Centering bells and edge grinding for the crown and flints, ground to very closely matched diameter. Glue applied, and the lenses installed in the tight-fitting barrel while cement is still wet.
2) Centering bells and edge grinding close to final diameter. Flint set on a precisely leveled flat surface. Crown floated on flint with cement. Gravity centers the lenses. After set, edge grind both to final diameter.
3). You slap them in like Turner Reich did, and damn the wedge error.

(I am a lens designer as well.. have been for a few decades and did a lot of gands-on optical shop work. I now am engineering manager at LaCroix Precision Optics. I also do other things like repair delamination at home for fellow photographers).

[Randy]

Once upon a time I acquired an old 12" rapid rectilinear - cost me nothing. It had some bad balsam separation. I removed the separating cell, let it soak in a dish of acetone, checking the progress of the dissolving balsam every day, and about a week later I could very easily separate the elements. After cleaning the residue off and washing both elements well, I put one drop of canola oil on one element, sandwiched them together, gently rocking to work out any bubbles, wiped the edges with a paper towel, reinstalled the cell into the barrel, made some water-house stops, and used the lens like that for a year or so, then sold it (fully disclosing my very inexpensive repair to the buyer).

[reddesert]

Internet references say vegetable oil has an index of refraction n about 1.47, so that would be a pretty good match to common glasses and a decent choice for a layer between lens elements from an optics point of view. I have never been in a position to consider such a repair; if I did, I might consider using mineral oil (the stuff you can buy to use on cutting boards and wood utensils), as vegetable oil might eventually go rancid.

[Randy]

I might consider using mineral oil (the stuff you can buy to use on cutting boards and wood utensils), as vegetable oil might eventually go rancid.

Yes, vegetable oil will likely turn bad. If I had kept the lens my plan was to remove the cell every so often, clean and replace the drop of canola oil. The last thing I want is for my lenses to start stinking.

[Mark J]

There are oils used for immersion microscope objectives, which would be ideal for this job if you can find some cheaply. The index is not too critical, as long as it's over approx 1.45. The layer between the lenses is so thin that the refractive effect of the oil is tiny once it's in place.

[Tin Can]

Does anybody polish UN coated lenses

with food grad activated charcoal

?

[cowanw]

I have a Rodenstock 105mm f/5.6 whose front element had separated about half way across. I probably should have left it but I soaked the front assembly and dissolved the cement, cleaned it and popped it back in waiting for the day I access some Canada Balsam. I can't tell the difference from my other enlarging lenses either when it was half separated or now with an air space.

[Myriophyllum]

Does anybody polish UN coated lenses

with food grad activated charcoal

?

Hi,

my guess: Too soft to be abrasive at all.

Jens