Repolishing old lenses with fungus, balsam separation

[Jody_S]

Just read through the whole thread, matches my personal experience. Comments:

1. Easiest way of separating any lens group, whether Canada Balsam or any lens cement I've ever encountered, is old furniture stripper (MEK-based). Hard to find, I scour flea markets for old cans.

2. Centering most lens groups is easy with a flat surface and a pair of machining V-blocks.

3. If you can't find UV-cure optical cement (I ordered mine off AliExpress), clear-coat UV-cured nail polish is a good substitute, available at any nail salon.

4. The only reason to polish off surface etching from fungus is if you plan to sell the lens, IMHO, and if it's a coated lens and you polish off the coating to make it look pretty, you might get a better price for your lens but it's a little unethical IMHO. I expect all those 'mint' Leica lenses for sale on FleaBay have had some treatment of this sort. If the etching is bad enough to affect a LF lens, then your grinding might manage to keep the exact curvature of the lens, if you follow the proper pitch lap technique, but changing the thickness of the lens by polishing off a huge amount of glass might affect the lens' performance anyway. Even more so if it's a modern coated lens. I don't mind doing this to polish off a huge number of cleaning marks.

[kfed1984]

The only reason to polish off surface etching from fungus is if you plan to sell the lens, IMHO, and if it's a coated lens and you polish off the coating to make it look pretty, you might get a better price for your lens but it's a little unethical IMHO. I expect all those 'mint' Leica lenses for sale on FleaBay have had some treatment of this sort. If the etching is bad enough to affect a LF lens, then your grinding might manage to keep the exact curvature of the lens, if you follow the proper pitch lap technique, but changing the thickness of the lens by polishing off a huge amount of glass might affect the lens' performance anyway. Even more so if it's a modern coated lens. I don't mind doing this to polish off a huge number of cleaning marks.

The polishing process will not remove more that a micron off the lens surface, which will not affect the optical performance. No I am not polishing to make it look "pretty", but to make the image sharp again. Fungus and microscratches make a diffused image. I wrote at the beginning; the decision to polish or not depends on the degree of damage and what the lens is worth to you. I had this damaged lens (the Apo-Ronar), shown in the example pictures, and it was worth for me to repair. Next example will be another damaged Apo-Gerogon lens, which I will show later, with lots of cleaning marks. But maybe polishing my own lenses will be unethical, I don't know.

The idea of using pitch is to preserve the curvature of original glass as done in the factory, not to make it "pretty". I don't think the "mint" lenses on eBay you mentioned had this procedure.

I don't think re-boring and re-honing the cylinders on your worn-out engine is unethical. Yeah, you may sell it as a low mileage engine, which may be unethical. But re-boring a car engine on its own is a good thing without doubt, and always good to learn how to do it for those who want to.

[Jody_S]

The polishing process will not remove more that a micron off the lens surface

You can polish off as much as you want, just by using a variety of grits and spending as much time as you want. You most definitely can change the curvature doing so, particularly if you use a heavier grit to polish out deep scratches and don't follow the proper pattern of movement. The amateur telescope-making book mentioned in the thread gives a variety of movement patterns to use, IIRC, as well as instructions for cutting and polishing a glass blank into a concave mirror.

I am not accusing you of doing anything unethical, but I have had many hundreds of lenses pass through my hands and I have spotted a number that have been imperfectly polished and re-sold. I very nearly bought a collapsible Summicron in LTM a few years back, from what I thought was a reputable photo dealer here in Montreal, only to notice on close inspection that an inner element had been mangled this way. So there are unethical people out there, and if you buy old and valuable lenses, this is something you should look for. I am skeptical of all those Universal Heliars and the like on FleaBay with 'perfect' glass. I am more inclined to take my chances on a lens with a few visible blemishes. Same reason I won't buy a Turner-Reich Triple Convertible that doesn't have separation.

[kfed1984]

You can polish off as much as you want, just by using a variety of grits and spending as much time as you want. .

Well I tried to do this as much as possible in agreement with the telescope making book. Also we should keep in mind that the accuracy required for making mirrors is about 4x more stringent than lenses, due to the nature of refraction vs. reflection. So should be ok. The special polishing strokes that are more concentrated on the mirror center and specially formed pitch laps designed to work more on the center or perimeter are designed for generating parabolas and hyperbolas, etc. In our case, most of the lenses are spherical, and to guarantee the spherical shape the tool and the lens need to be rotated periodically. The repolishing does not take too much time, about 30 min, again depending on the damage, so it should not dig too deep into the glass. For the lens shown in my photo it took about 30 min per side. The grit is also quite small so we are not grinding anything, but polishing. Although polishing is basically grinding on a sub-micron scale.

I think those lenses you are taking about from ebay have been buffed on an industrial buffing wheel. I doubt somebody will go through the trouble of forming the lap, etc.

The whole exercise here is to try to use the same manufacturing techniques as used to make the lens. If you look at the pictures of industrial polishing setups from 1960's, they look quite rudimentary. Just need to follow some rules to ensure the law of averages of the polishing strokes generates a spherical surface than something else.

For the next lens I will try to do an image quality analysis at the center and corners before and after polishing.

[pgk]

The whole exercise here is to try to use the same manufacturing techniques as used to make the lens.

FWIW. It may have been Twyman's (Hilger & Watts) book in which the author states that 200 grinding rotations with the 'finest grit' would remove about 1/4 wavelength of depth from a lens surface . As neither glass type nor specific wavelength was mentioned I think it was simply an illustrative suggestion as to how much polishing is required to remove a small amount of a hard material like glass.

[kfed1984]

FWIW. It may have been Twyman's (Hilger & Watts) book in which the author states that 200 grinding rotations with the 'finest grit' would remove about 1/4 wavelength of depth from a lens surface . As neither glass type nor specific wavelength was mentioned I think it was simply an illustrative suggestion as to how much polishing is required to remove a small amount of a hard material like glass.

I think this is referring to the grinding operation with the smallest emery grit, which is different from polishing. In grinding telescope mirrors for example, the grit is rolled back and forward between two glass surfaces in contact; the glass tool and the mirror. In the case of polishing, the maximum pressure each particle can exert is limited by the pitch plasticity. Exerting more pressure on the tool, forces the grit into the pitch, which places a limit on the stress each particle can exert on the lens surface. Whereas in grinding, the particles are crushed between two pieces of glass, creating large stresses that breaks down the glass.

There is no way that in polishing a telescope mirror you will remove 1/4 wavelength in 200 strokes, which will be couple minutes of work. Correcting a 1/10 wavelength error will probably take an hr of work or more.

[kfed1984]

FWIW. It may have been Twyman's (Hilger & Watts) book in which the author states that 200 grinding rotations with the 'finest grit' would remove about 1/4 wavelength of depth from a lens surface . As neither glass type nor specific wavelength was mentioned I think it was simply an illustrative suggestion as to how much polishing is required to remove a small amount of a hard material like glass.

Found a link to your book, downloadable:
https://archive.org/details/20210902171836226

[kfed1984]

and I just found this statement in Chapter III. I will probably order this book, this is just what I was looking for. Thanks for sharing.

[Mark J]

There is no way that in polishing a telescope mirror you will remove 1/4 wavelength in 200 strokes, which will be couple minutes of work.

Well, I disagree, that sounds plausible for polishing with pitch ( grinding with fine grit would remove more ) if the glass is soda-lime eg. float glass.
ps. I've made about 6 telescope mirrors.
It may be that the figure would not change much in 200 strokes ( if you were just using a well-running lap and normal strokes ) but that doesn't mean that material isn't being removed.
You can change the figure by 1/10 wave easily in a few dozen strokes if you apply a more extreme stroke, I've seen this several times.

If you are polishing Pyrex/Duran, then the removal would be lower, maybe only half this.

[kfed1984]

Well, I disagree, that sounds plausible for polishing with pitch ( grinding with fine grit would remove more ) if the glass is soda-lime eg. float glass.
ps. I've made about 6 telescope mirrors..

Is there something like a Foucault test for lenses? How do you test and refigure a lens?