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Lens surface rehab.
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PostPosted: Sat Feb 26, 2022 9:12 pm    Post subject: Lens surface rehab. Reply with quote
I mentioned in a previous thread on the aperture rebuild, I would discuss my "as-is" xenotar objective surface issues. I bought it with the thought that the surface just looked dirty . A good cleaning and I d have a usable copy of a rare and legendary optic. Nope. The surface is pretty bad. It appears the coating is etched. I did my usual wash with isopropyl to no avail. Then scrubbed with vinegar, nope. Mild bleach? Nope. Tried lighter fluid. Even acetone. Finally I tried automotive rubbing compound on one edge. No noticeable improvement. At this point I'm willing to sacrifice the coating for a usable lens. Ideas?


PostPosted: Sat Feb 26, 2022 9:17 pm    Post subject: Reply with quote


PostPosted: Sat Feb 26, 2022 10:25 pm    Post subject: Reply with quote
Following this thread with interest. I have the same problem with a Minolta mc 50 1,4 PG.


PostPosted: Sun Feb 27, 2022 12:22 pm    Post subject: Reply with quote
I understand cerium oxide would remove coating and good for polishing glass , there are different qualities never tried it though


PostPosted: Mon Feb 28, 2022 1:42 pm    Post subject: Reply with quote
kiddo wrote:
I understand cerium oxide would remove coating and good for polishing glass , there are different qualities never tried it though


You would first need to make a polisher though; you would want to maintain the correct curvature of the glass surface, which would easily be destroyed by polishing off the coating freehand.

In the optics industry they used to press the lens into pitch to create a polisher, and then score it for the polishing slurry. Quite a bit of experience required to do that.

Personally, I would consider this lens beyond DIY improvement.


PostPosted: Mon Feb 28, 2022 4:28 pm    Post subject: Reply with quote
I have nothing to lose but time at this point.


PostPosted: Mon Feb 28, 2022 4:43 pm    Post subject: Reply with quote
I use cerium oxide paste on filters which have 'oxidized' after exposure to air for too long. I got mine from UVIRoptics - https://www.ebay.com/usr/uviroptics - as apparently there are different grades of it. This should be able to remove the coating and not damage the glass (in theory at least).

It's not listed on his ebay site at the moment, but it is worth contacting to see if he has any.


PostPosted: Tue Mar 01, 2022 3:45 am    Post subject: Reply with quote
I've had excellent results by spraying the lens with a clear lacquer coat. Just a quick, thin coating spray and it fills-in all the gaps in the coating and leaves a smooth surface. The surface won't hold up to much cleaning, as the lacquer is quite soft and scratches easily, but it is easy to re-apply. Plus, if you get it on too thick, it's easy to clean off with acetone or lacquer thinner and try again. Has worked on some high power / high NA objectives with good results as well. Low hanging fruit.

Additive, and reversible, vs subtractive, and non-reversible.


PostPosted: Tue Mar 01, 2022 5:36 am    Post subject: Reply with quote
Just curious: how’s the image quality with this bad front lens?


PostPosted: Tue Mar 01, 2022 10:50 am    Post subject: Reply with quote
Ray Parkhurst wrote:
I've had excellent results by spraying the lens with a clear lacquer coat. Just a quick, thin coating spray and it fills-in all the gaps in the coating and leaves a smooth surface. The surface won't hold up to much cleaning, as the lacquer is quite soft and scratches easily, but it is easy to re-apply. Plus, if you get it on too thick, it's easy to clean off with acetone or lacquer thinner and try again. Has worked on some high power / high NA objectives with good results as well. Low hanging fruit.

Additive, and reversible, vs subtractive, and non-reversible.


An interesting option. If it does result in a usable lens then a clear/UV filter will sort out the required protection.


PostPosted: Tue Mar 01, 2022 6:38 pm    Post subject: Reply with quote
Ray Parkhurst wrote:
I've had excellent results by spraying the lens with a clear lacquer coat. Just a quick, thin coating spray and it fills-in all the gaps in the coating and leaves a smooth surface. The surface won't hold up to much cleaning, as the lacquer is quite soft and scratches easily, but it is easy to re-apply. Plus, if you get it on too thick, it's easy to clean off with acetone or lacquer thinner and try again. Has worked on some high power / high NA objectives with good results as well. Low hanging fruit.

Additive, and reversible, vs subtractive, and non-reversible.
I thought of trying something like this as a friend of mine used Johnson wax and buffed out degraded surfaces of vintage CD's for gaming. I will try it and report back.


PostPosted: Tue Mar 01, 2022 7:19 pm    Post subject: Reply with quote
Sounds like a plan!

For reference, Zeiss uses cigarette ash instead of fine cerium oxide. Ref:

http://forum.mflenses.com/ash-polishing-glass-t16384.html
http://forum.mflenses.com/hazy-lens-element-not-responding-to-normal-cleaning-methods-t66075.html


PostPosted: Tue Mar 01, 2022 9:10 pm    Post subject: Reply with quote
visualopsins wrote:
Sounds like a plan!

For reference, Zeiss uses cigarette ash instead of fine cerium oxide. Ref:

http://forum.mflenses.com/ash-polishing-glass-t16384.html
http://forum.mflenses.com/hazy-lens-element-not-responding-to-normal-cleaning-methods-t66075.html


It may well be true, but does anyone have an actual reference to such statement by Zeiss themselves?

The only historic supposed link I could find is now dead.

I would like to know what they really said about it.


PostPosted: Tue Mar 01, 2022 10:26 pm    Post subject: Reply with quote
There are some great YouTube videos covering the making of optics for astronomy telescopes, the making of a polishing lap and the process involved, I'm sure some of the astronomy DIY'ers would help direct you in the right direction to get a nice smooth and correct(or close to correct) surface geometry.
YT Channels I've watched that had some info:
Gordon Waite
Huygens Optics - Why is this space telescope so tiny?

I do know that you can cast a mold of an optic that's not too damaged to make a lap for polishing that optic, cover/stretch cling wrap over the lens and press it into silicon or other curable compound that doesn't get hot. It may take 2-3 attempts to get a clean lap with no distortion or raised lip on the rim. Then comes the slow and tedious polishing part... Your lens didn't look too far gone, still looked shiny in spots, that means it shouldn't take too much effort to restore the surface and you won't have to be as aggressive.

This will get you an uncoated lens surface, one surface shouldn't have too big of an impact unless it's a critical surface(some lens designs have some surfaces that will transmit reflections more than with other designs(mostly in wider angle lenses), for example Canon's nano coating is only on these critical surfaces and use SSC on other surfaces, this is probably because the nano coating is so delicate in comparison.


PostPosted: Tue Mar 01, 2022 11:50 pm    Post subject: Reply with quote
RokkorDoctor wrote:
visualopsins wrote:
Sounds like a plan!

For reference, Zeiss uses cigarette ash instead of fine cerium oxide. Ref:

http://forum.mflenses.com/ash-polishing-glass-t16384.html
http://forum.mflenses.com/hazy-lens-element-not-responding-to-normal-cleaning-methods-t66075.html


It may well be true, but does anyone have an actual reference to such statement by Zeiss themselves?

The only historic supposed link I could find is now dead.

I would like to know what they really said about it.


What is the historic link you have, please?

Used to be on this page iirc: https://www.zeiss.com/consumer-products/us/service/content/fungus-on-lenses.html

Somebody else saw it too: http://forum.mflenses.com/canon-fd-15mm-f2-8-fisheye-ssc-heavy-problem-t25320,highlight,canon.html

Quote:
... the Carl Zeiss page, and there said i could use cigar ash to polish it. I was polishing it with ash and cotton (as the page said)...


PostPosted: Wed Mar 02, 2022 9:35 am    Post subject: Reply with quote
visualopsins wrote:

What is the historic link you have, please?

Used to be on this page iirc: https://www.zeiss.com/consumer-products/us/service/content/fungus-on-lenses.html

Somebody else saw it too: http://forum.mflenses.com/canon-fd-15mm-f2-8-fisheye-ssc-heavy-problem-t25320,highlight,canon.html

Quote:
... the Carl Zeiss page, and there said i could use cigar ash to polish it. I was polishing it with ash and cotton (as the page said)...


That is the same link I saw, but Zeiss seem to have removed that page. Shame, would have liked to have seen the suggested procedure.

A bigger issue is that I don't smoke, so would be good to know if there is something special about cigarette ash, or whether e.g. wood ash would also work.


PostPosted: Wed Mar 02, 2022 10:48 am    Post subject: Reply with quote
RokkorDoctor wrote:
I don't smoke, so would be good to know if there is something special about cigarette ash, or whether e.g. wood ash would also work.


The most obvious difference is that cigarette ash doesn't contain any bigger pieces whereas normal wood ash may contain bigger and hard ones which may again scratch the surface.


PostPosted: Wed Mar 02, 2022 1:03 pm    Post subject: Reply with quote
Sadly the earliest save here is Aug 2020: https://web.archive.org/web/2021*/https://www.zeiss.com/consumer-products/us/service/content/fungus-on-lenses.html
And the forum post is 11 years earlier, my guess is someone following their directions and demanded they fix or replace their lens which was perfect prior to "Polishing" it.

As to the process?
From this link: http://forum.mflenses.com/ash-polishing-glass-t16384.html
Quote:
Smoke a cigarette, drop the ashes on a clean tray (and don't let the cigarette rest in there, or a puddle of tar will deposit), dab a clean polishing cloth into the ashes, and polish the lens with light pressure and circular movements. Try on some damaged or useless lens first, to get a feeling for the right pressure and even movements.

Seems fairly descriptive to me, basically dab the ash on a cloth like you are applying makeup and loading up a sponge, the ash should crush down to a soft powder between your fingers, it should not feel gritty!!!!

You probably have a few options to try, look for the softest cloth you can find, microfibre cloth should be safe after testing it first on something similar(bare fingers may also work, you would feel any grit instantly) , preferably a bit softer, test the cloth by its selfie a marked off quadrant, then with just the ash, then you can try some different lubricants, alcohol, water, and oil, see which one works best, does how wet the slurry is effect the results? And pressure?

Test first, then go slow and light till you can judge how soft the lens is and how abrasive the ash is, as the abrasive breaks down it will become softer/finer. I'd rather spend 10x as long using 1/5th the pressure to get a 10% improvement vs. too much pressure and over do it.

When polishing, try to avoid prolonged repetitive motions, mix it up a bit, for example do a motion a few times then rotate the element by 80 degrees do the same number of motions, another 80 degrees repeat.... then change the motion and rotate by 50 degrees... and then a different motion and a different rotation amount that doesn't divide equally into 360 degrees. This will help eliminate any bias to your polishing technique, keep in mind the centre will likely get too much attention so the first motion is a strait line in the centre front to back3 times then rotate, 3 times then rotate, repeat 6-8 times then move the centre line to the right, half way between the centre and edge, do that 6-8 times, then move to the right and do the edge 6-8 times, then clean the lens and inspect the damage, look for any abrasive patterns in the lens.


PostPosted: Wed Mar 02, 2022 6:17 pm    Post subject: Reply with quote
I will try the lacquer or wax thing first but if that fails I have purchased some cerium oxide optical powder. I have a silicone mold that should be cured by now. (Lens; saran wrap; silicone puddle; dremel bit) I will report back at each attempt.


PostPosted: Tue Apr 19, 2022 12:36 pm    Post subject: Reply with quote
I did test the Zeiss recommendation, without a good result on an Canon nFD lens.

Later I made som experiments with a car window polsih paste, whiich works quite good, even on relative modern lenses:
http://www.4photos.de/camera-diy/Lens-Coating-Removal.html
http://www.4photos.de/camera-diy/Photo-Lens-Scratch-Repair.html

But as a warining, better don´t use that, when there culd be radioactive ingediens in the lens or coating. And the lens is without coating, and will have at least a little bit different surface form and quality.


PostPosted: Tue Apr 19, 2022 8:34 pm    Post subject: Reply with quote
jamaeolus wrote:
I will try the lacquer or wax thing first but if that fails I have purchased some cerium oxide optical powder. I have a silicone mold that should be cured by now. (Lens; saran wrap; silicone puddle; dremel bit) I will report back at each attempt.


I plan to try something like this to remove a damaged / hazed coating. I have got 3 grades of cerium oxide, I don't know what they are, I was given then by someone who makes glass jewellery.
For a mould I thought I would try Sugru mouldable silicone, or maybe a two part epoxy putty like Milliput.

https://sugru.com/

https://www.milliput.com/

The Sugru would be more flexible than the Milliput, maybe one will be better than the other? I don't know, I haven't had a chance to experiment yet.
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