Strange Whiteness

Bosun Higgs · Sep 1, 2022

Over the years I've occasionally come across lenses with what looks like internal fungus, but isn't.

There is a thin white surface layer that diffuses light, but there are no mycelia under a microscope. The layer is resistant to alcohol and solvents and is unaffected by an NH4OH/H2O2 bath.

Most of the lenses have only one surface of one element affected, but in my most recent case, a big triplet, the inner surfaces of both the outer elements were affected.

So far, I have only seen this problem in coated lenses and all of these were projection lenses.

So, I'm thinking intense focused heat somehow degrading the coating?

I have never seen this effect in a Petzval, only 6 element Double Gauss derivatives where it affects a "central" element, and tripets, where the outer element(s) are affected.

When wet or damp the element looks clear, but as soon as it dries the matte whiteness reappears.

If this is, as I suspect, degraded coating, is there a way to polish this off?

Modern AR multicoats are only 200-300nm thick, so it should not take much to remove this (presumably) single coating, has anyone here tried to polish off a damaged coating?

Aoi Usagi · Sep 1, 2022

Bosun Higgs said:
Over the years I've occasionally come across lenses with what looks like internal fungus, but isn't.

There is a thin white surface layer that diffuses light, but there are no mycelia under a microscope. The layer is resistant to alcohol and solvents and is unaffected by an NH4OH/H2O2 bath.

Most of the lenses have only one surface of one element affected, but in my most recent case, a big triplet, the inner surfaces of both the outer elements were affected.

So far, I have only seen this problem in coated lenses and all of these were projection lenses.

So, I'm thinking intense focused heat somehow degrading the coating?

I have never seen this effect in a Petzval, only 6 element Double Gauss derivatives where it affects a "central" element, and tripets, where the outer element(s) are affected.

When wet or damp the element looks clear, but as soon as it dries the matte whiteness reappears.

If this is, as I suspect, degraded coating, is there a way to polish this off?

Modern AR multicoats are only 200-300nm thick, so it should not take much to remove this (presumably) single coating, has anyone here tried to polish off a damaged coating?

I don't know what the whiteness you are referring to, but the heat would need to be extremely high to burn off lens coatings.

As for DIY polishing, I have done it before in this thread .

It was a big improvement, but not perfect by any means. It was more of a experiment on a lens that wasn't worth anything.

ProfHankD · Sep 1, 2022

Bosun Higgs said:
There is a thin white surface layer that diffuses light, but there are no mycelia under a microscope. ...

When wet or damp the element looks clear, but as soon as it dries the matte whiteness reappears.

Basically, it sounds like you are describing an element that has been etched. That can be what remains after cleaning a bad fungus infection, or it can happen if somebody cleans too aggressively essentially grinding the glass or coating.

If so, there really isn't a legitimate fix, although, as you observed, wetting the surface can smooth it. A small amount of grease or oil wiped on the surface can resist drying for a long time. In fact, this is also a well known old fix for scratches: filling the scratch this way prevents diffraction and rainbow artifacts from even fairly deep scratches. It's a little gross, but grease from the side of one's nose used to be recommended to fill scratches in negatives for printing. I suspect a clear gloss polyurethane paint coating rubbed on would also work -- sort of the same idea as the little kits they sell for setting clear plastic in a nick or pit in a car's glass windshield, but at the microscopic scale.

fferreres · Sep 1, 2022

Bosun Higgs said:
Over the years I've occasionally come across lenses with what looks like internal fungus, but isn't.

There is a thin white surface layer that diffuses light, but there are no mycelia under a microscope. The layer is resistant to alcohol and solvents and is unaffected by an NH4OH/H2O2 bath.

Most of the lenses have only one surface of one element affected, but in my most recent case, a big triplet, the inner surfaces of both the outer elements were affected.

So far, I have only seen this problem in coated lenses and all of these were projection lenses.

It's also seen many other lenses, including earlier Summarit and Summicron lenses. They get haze in the inner elements, and the lab trying to clean that haze can't. Why the inner elements? We know are 1) close to the iris, and thus close to oil 2) more exposed to humidity/air flow, thus deposits and formation of haze by residue and deposit.

I also read anecdotal comments that if left hazy for long, chances are it won't clear. Corrosion of the coating is thus more likely. But so the outer elements. What it doesn't have is the enclosed space and more contact with the gassing of oil. Another aspect is light would be more intense in that area, prone to haze.

Bosun Higgs said:
So, I'm thinking intense focused heat somehow degrading the coating?

Yes, but also factor in the projectors also operate for hours, can be in humid places and have large fans to keep temperatures below melting levels. This increase in airflow would also cause deposit more quickly, and the fact these may operate for hours and hours non stop, in hot conditions, is also something particular to projectors. Now combined, there are additional factors, like significant haze almost never being corrected in these units, but that this haze will also trap and generate heat as light is absorbed. Or it could contribute to fostering corrosion of the coating.

here's one of the most expensive I found on Amazon, to give you an idea, some other very cheap (and Amazon as just a reference of the kind of tool used).

3M Glass Polishing Compound, 60150: Cerium Oxide: Amazon.com: Industrial & Scientific

www.amazon.com

Then there are some very cheap compounds. If more of a perfectionist, there are many different sizes of each polishing once you start reading about it.

Bosun Higgs said:
I have never seen this effect in a Petzval, only 6 element Double Gauss derivatives where it affects a "central" element, and tripets, where the outer element(s) are affected.

Puzzling. Any difference between double gauss with iris vs those without? I can't think why a Petzval would not see corrosion, triplet outside and a Gauss inside.

Bosun Higgs said:
When wet or damp the element looks clear, but as soon as it dries the matte whiteness reappears.

If this is, as I suspect, degraded coating, is there a way to polish this off?

Modern AR multicoats are only 200-300nm thick, so it should not take much to remove this (presumably) single coating, has anyone here tried to polish off a damaged coating?

Report back if you try. I read not all polish is the same and to decant it for extra precaution, as some particles may be bigger. There's a big difference between the same "material" such as carbide oxide and the cheap one will have more variance in the particles, or have larger particles.

I think the quality of the polishing compound (low variance, and actual size of average particle) can have an important effect, but also what is used to make with the glass, as the outer region moves a lot faster, but the haze may be uneven, more in the center, sometimes more in the periphery...

My only experience with polishing are those 3M kits for car lights that loose transparency due to corrosion from sunlight and exposed to the environment, as well as (quite unimpressive) rotary tumbling when my son was 5 or 6.

Bosun Higgs · Sep 2, 2022

I don't think either oil or fungus are the culprits here.

The lenses are all projection optics, they have no iris and therefore no lubricants of any kind inside to cause haze.

The last lens still had the "paint blob" seals on the retaining ring, I had to chip these away to get in. Also the paint finish was very delicate and the spanner slots and the threads themselves were unmarked, I'm pretty certain I was the first one to disassemble it. I've seen umpteen fungus infected lenses over the years and this just didn't look like that, that's why I popped an element under the microscope - no fungus structure.

With untouched fungus there is always something that will just wipe off, these coatings are unchanged even after solvent and chemical treatment.

It is also the precisely targeted nature of the effect that is odd, one Benoist lens had eight elements with a single cemented doublet right in the centre, only one face of the doublet was affected and the white layer was so perfect it looked like a factory finish. The optical surface facing it just millimetres away had pristine coating.

Whilst not knowing the causal mechanism is tantalizing, a fix is more important. After seeing the water clarifying I tried a range of oils and greases with poor results. I suspect that the surface tension of the water film is the source of its magic effect.

I'm going to try the grinding route, so if anyone has seen a detailed account of successful coating removal anywhere, I'd appreciate any info, especially anything on the best grades of Cerium Oxide to use.

ProfHankD · Sep 2, 2022

Bosun Higgs said:
Whilst not knowing the causal mechanism is tantalizing, a fix is more important. After seeing the water clarifying I tried a range of oils and greases with poor results. I suspect that the surface tension of the water film is the source of its magic effect.

That's very strange; oil/grease usually works fine for this sort of thing...

Bosun Higgs said:
I'm going to try the grinding route, so if anyone has seen a detailed account of successful coating removal anywhere, I'd appreciate any info, especially anything on the best grades of Cerium Oxide to use.

Good luck. You're entering the I-wouldn't-try-that-unless-the-lens-is-unusable zone. ;-)

Bosun Higgs · Sep 2, 2022

ProfHankD said:
Good luck. You're entering the I-wouldn't-try-that-unless-the-lens-is-unusable zone. ;-)

Well, fortunately these lenses all came at bargain basement (sub £10) prices because of their "milkiness". They are unusable in their present state so I'm not losing much if the grinding is not successful.

I quite often buy lenses in this state as I am an old hand at fungus/haze removal and I have got quite a few bargains this way. I usually end up stripping most lenses I buy anyway when the internal dust specks and debris get too much for my inner picture straightener, even though they don't affect IQ.

E Dinkla · Sep 2, 2022

Quite a complex Benoist lens then if it has 8 elements. I wonder whether it might not be glass but a fluorite, a plastic or at least a less inert glass quality. Was it not in the Canon LTM 50mm lenses that they suffered of a glass that was easily affected.

Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

Bosun Higgs · Sep 2, 2022

E Dinkla said:
Quite a complex Benoist lens then if it has 8 elements. I wonder whether it might not be glass but a fluorite, a plastic or at least a less inert glass quality.

I have never heard of fluorite or exotic glasses being used in projection lenses. Bear in mind, most of my lenses date from 1960 or earlier, most don't even have coatings. I think that the Benoist is an early 60's vintage.

The lens was made for a Debrie D-16 projector, although only a 16mm machine it was huge, it weighed nearly 50kg.

Both lenses are for 16mm cine, but the Benoist has obviously been hitting the steroids.

The lens is the same, massive for a 16mm cine lens, the picture shows it beside a 2 inch Taylor Hobson Petzval also for 16mm cine. Most projection lenses are brass bodied under the plating and I thought that Benoist was too as it was very heavy, but the body is actually lightweight aluminium alloy, the weight comes from all the glass they have stuffed inside.

Despite the size the Benoist vignettes on APS-C, so it must squeeze as much light as possible down to the 16mm frame, mind you with eight elements and single coating, I dread to think what the T-stop is.

This is the only 8 element projection lens in my collection, even the neutron-star material heavy Gaumont Kalee Series S Double Gauss variants only have six, but as they cover 70mm, they are huge.

The doublet in the Benoist is one of those weird longer-than-it-is-wide types, I've only seen these before in the Bolex range, more of a light-pipe than a lens. Maybe it's a French/Swiss thing, with the edge blackening, they look like short lipstick tubes.

I just hope I can get it fettled enough to shoot with.

fferreres · Sep 2, 2022

Bosun Higgs said:
I don't think either oil or fungus are the culprits here.

The lenses are all projection optics, they have no iris and therefore no lubricants of any kind inside to cause haze.

The last lens still had the "paint blob" seals on the retaining ring, I had to chip these away to get in. Also the paint finish was very delicate and the spanner slots and the threads themselves were unmarked, I'm pretty certain I was the first one to disassemble it. I've seen umpteen fungus infected lenses over the years and this just didn't look like that, that's why I popped an element under the microscope - no fungus structure.

With untouched fungus there is always something that will just wipe off, these coatings are unchanged even after solvent and chemical treatment.

It is also the precisely targeted nature of the effect that is odd, one Benoist lens had eight elements with a single cemented doublet right in the centre, only one face of the doublet was affected and the white layer was so perfect it looked like a factory finish. The optical surface facing it just millimetres away had pristine coating.

Could it be separation? Is the whiteness circular and in the center? If you have picture, we can get sense of what you are describing with words.

Bosun Higgs said:
Whilst not knowing the causal mechanism is tantalizing, a fix is more important. After seeing the water clarifying I tried a range of oils and greases with poor results. I suspect that the surface tension of the water film is the source of its magic effect.

I'm going to try the grinding route, so if anyone has seen a detailed account of successful coating removal anywhere, I'd appreciate any info, especially anything on the best grades of Cerium Oxide to use.

I've seen polishing more often in astro aficionado forums than in lens taking. I have not seen any resource that would be good enough to recommend, and have not polished any lens. The cases I've seen, there's a few ours of polishing.

ProfHankD · Sep 2, 2022

fferreres said:
The cases I've seen, there's a few ours of polishing.

I've only polished a few lenses. They were homemade, 3D-printed plastic, polished with a custom-printed jig for my drill press -- which works rather like the spherical lens polishing rigs really used in lens production. Despite that, it was closer to taking days per surface than hours. Of course, YMMV, and you are talking about polishing a glass lens that had already been polished, so hopefully you don't have as far to go...

Bosun Higgs · Sep 5, 2022

Well, I've done some research on coating removal and my mind is truly boggled.

There are folk out there (mainly in the video world) that take perfectly good camera lenses and attempt to remove the coatings to get a "filmic" look. This seems to mean lens flare turned up to 11 with every take. Contrast is naturally totally shot, and marked image colour changes seem to be particularly valued.

The removal methods used to obtain said "filmic" look bordered on the bizarre, including, but not limited to, Brasso metal polish, spirit vinegar, car windshield scratch paste, cigarette ash, and Hydrofluoric Acid derivatives (bone cancer anyone?).

I'm thinking Cerium Oxide sounds like a better option, it is used for final polishing on glass and is very slow, so I hope this will allow some degree of fine control.

Luckily all of the surfaces I have to polish are flat, so it shouldn't be too hard to find something to use as a lap, a sheet of aluminium or float glass perhaps?

Anyway the CeO2 is on order and....... We Shall See.

Tons o Glass · Sep 5, 2022

Increased moisture over a prolonged period of time may affect some coatings. If that moisture is slightly acidic it could etch the surface, and some lens coating materials are ever-so-slightly soluble in water especially as the temperature increases. So I think it could be a combination of moisture, acidity, and/or heat.

It's also possible for an ultrasonic cleaner to damage some lens coatings . I would imagine the ultrasonic cleaner is greatly accelerating whatever mechanisms cause lens coating haze under normal circumstances.

I have a Canon FL 200/3.5 that had water droplets and fog trapped internally for who knows how long. I opened it up to dry it out, which went fine, but the fog didn't go away with any of the usual cleaning methods; it was in/under the coating, with structure that resembles the water droplets I removed.

As for polishing lenses, I've only tried the following once on a lens that was a lost cause; Ultrabrite toothpaste. It ultimately improved the lens, but I doubt it looks nice under a microscope.

fferreres · Sep 5, 2022

Bosun Higgs said:
Well, I've done some research on coating removal and my mind is truly boggled.

There are folk out there (mainly in the video world) that take perfectly good camera lenses and attempt to remove the coatings to get a "filmic" look. This seems to mean lens flare turned up to 11 with every take. Contrast is naturally totally shot, and marked image colour changes seem to be particularly valued.

The removal methods used to obtain said "filmic" look bordered on the bizarre, including, but not limited to, Brasso metal polish, spirit vinegar, car windshield scratch paste, cigarette ash, and Hydrofluoric Acid derivatives (bone cancer anyone?).

I'm thinking Cerium Oxide sounds like a better option, it is used for final polishing on glass and is very slow, so I hope this will allow some degree of fine control.

You can also research a wealth of information in stone tumbling sites. Glass is of very different characteristics, so different glass materials could be significantly different. They are classified by hardness, so could learn about the grit.

Actually, for example this one:

https://nanomaterialpowder.com/nano/silicon-carbide-sic-micron-powder-purity-99-5-size-5-25-μm/

Has most particles between 5 and 25 nm (microns). The larger ones weight 25 more than the smallest ones. That's the difference between 800 grit and 4000 grit, so you won't ever accomplish a lot more than 800 if you have particles that are big mixed with the smaller ones. It's like having a broad brush along a fine one, the broad one will always undo the fine traces.

Bosun Higgs said:
Luckily all of the surfaces I have to polish are flat, so it shouldn't be too hard to find something to use as a lap, a sheet of aluminium or float glass perhaps?

If really flat, can look into articles about making how quality ground glass. This one is flat although it's designed to not be very polished. I think a rotary tool (wheel based) with something like a sponge and doing a specific patterns would be better, because flat polishing will take a huge inordinate amount of time manually.

Bosun Higgs said:
Anyway the CeO2 is on order and....... We Shall See.

One interesting thing you could do is see if you have a lens that can image at 1:1 to 4:1. At 1:1 the size of the pixels match the size of the object (lens element). At 4x you have 16 pixels per size of the sensor pixel pitch (or is it 4?). If you use a 24MP FF you are around 8 nm (microns) in pixel (sensel) size.

If the grit size 5 to 25 microns this means you could sense how it looks with 1:1 and really image in detail with 4:1. If you can see what is doing the polishing, you'll also get a sense of what it will do to the glass surface. My point is you could image the surface to determine how this mystery white looks like, and then also image what the grit looks like, and what the effect of polishing is...just by taking some macro shots.

fferreres · Sep 5, 2022

Tons o Glass 0 Class said:
Increased moisture over a prolonged period of time may affect some coatings. If that moisture is slightly acidic it could etch the surface, and some lens coating materials are ever-so-slightly soluble in water especially as the temperature increases. So I think it could be a combination of moisture, acidity, and/or heat.

I think this is the fate of many lenses. I even have a summicron with the most misterious coating wear. It's not observable except for strong light from some angles, it just makes it into a Polka Dot surface of little areas that are a bit reflective and some aren't. So it must have come from something that condensed into liquid form at some point. Since it's one surface and maybe 70% of it still coated and quite evenly across the glass, it still performs very nicely.

Tons o Glass said:
It's also possible for an ultrasonic cleaner to damage some lens coatings . I would imagine the ultrasonic cleaner is greatly accelerating whatever mechanisms cause lens coating haze under normal circumstances.

I have a Canon FL 200/3.5 that had water droplets and fog trapped internally for who knows how long. I opened it up to dry it out, which went fine, but the fog didn't go away with any of the usual cleaning methods; it was in/under the coating, with structure that resembles the water droplets I removed.

As for polishing lenses, I've only tried the following once on a lens that was a lost cause; Ultrabrite toothpaste. It ultimately improved the lens, but I doubt it looks nice under a microscope.

I only polished car headlights, which is for soft plastic, but the feeling after polishing was so nice. I can imagine polishing a lens being a nightmare project, as it'd be like rock tumbling but where you don't want to affect the shape of glass, very small particles and not too fast.

Bosun Higgs · Sep 6, 2022

fferreres said:
You can also research a wealth of information in stone tumbling sites. Glass is of very different characteristics, so different glass materials could be significantly different. They are classified by hardness, so could learn about the grit.

Actually, for example this one:

https://nanomaterialpowder.com/nano/silicon-carbide-sic-micron-powder-purity-99-5-size-5-25-μm/

Has most particles between 5 and 25 nm (microns). The larger ones weight 25 more than the smallest ones. That's the difference between 800 grit and 4000 grit, so you won't ever accomplish a lot more than 800 if you have particles that are big mixed with the smaller ones. It's like having a broad brush along a fine one, the broad one will always undo the fine traces.

The SiC powder you mention is much more aggressive than the CeO2 I've ordered. Typically diamond and SiC powders are used for "stock removal" and CeO2 for final polishing.

I have no idea of the thickness of the White Curse (WC), modern multicoats are 200-300um thick and my hope is that the WC is degraded coating as this puts a limit on the thickness.

So my idea was to try the CeO2 first, if it is too slow, or if the WC is actually a thicker layer of etched glass, then I'll get SiO2. The CeO2 is quoted as 3.5um average size, so there is obviously a size distribution, but the powder is made specifically for glass polishing.

fferreres said:
One interesting thing you could do is see if you have a lens that can image at 1:1 to 4:1. At 1:1 the size of the pixels match the size of the object (lens element). At 4x you have 16 pixels per size of the sensor pixel pitch (or is it 4?). If you use a 24MP FF you are around 8 nm (microns) in pixel (sensel) size.

If the grit size 5 to 25 microns this means you could sense how it looks with 1:1 and really image in detail with 4:1. If you can see what is doing the polishing, you'll also get a sense of what it will do to the glass surface. My point is you could image the surface to determine how this mystery white looks like, and then also image what the grit looks like, and what the effect of polishing is...just by taking some macro shots.

I've had a look at the WC under a microscope, it has a smooth, very fine granular appearance, but as the material is glass it is viewed with both transmitted and reflected light which makes it difficult to see detail. I don't think macro shots wil improve on this.

fferreres · Sep 6, 2022

Bosun Higgs said:
fferreres said:

You can also research a wealth of information in stone tumbling sites. Glass is of very different characteristics, so different glass materials could be significantly different. They are classified by hardness, so could learn about the grit.

Actually, for example this one:

https://nanomaterialpowder.com/nano/silicon-carbide-sic-micron-powder-purity-99-5-size-5-25-μm/

Has most particles between 5 and 25 nm (microns). The larger ones weight 25 more than the smallest ones. That's the difference between 800 grit and 4000 grit, so you won't ever accomplish a lot more than 800 if you have particles that are big mixed with the smaller ones. It's like having a broad brush along a fine one, the broad one will always undo the fine traces.

Click to expand...

The SiC powder you mention is much more aggressive than the CeO2 I've ordered. Typically diamond and SiC powders are used for "stock removal" and CeO2 for final polishing.

I have no idea of the thickness of the White Curse (WC), modern multicoats are 200-300um thick and my hope is that the WC is degraded coating as this puts a limit on the thickness.

So my idea was to try the CeO2 first, if it is too slow, or if the WC is actually a thicker layer of etched glass, then I'll get SiO2. The CeO2 is quoted as 3.5um average size, so there is obviously a size distribution, but the powder is made specifically for glass polishing.

Let us know how it goes. What you say makes sense.

Bosun Higgs said:
Bosun Higgs said:

One interesting thing you could do is see if you have a lens that can image at 1:1 to 4:1. At 1:1 the size of the pixels match the size of the object (lens element). At 4x you have 16 pixels per size of the sensor pixel pitch (or is it 4?). If you use a 24MP FF you are around 8 nm (microns) in pixel (sensel) size.

If the grit size 5 to 25 microns this means you could sense how it looks with 1:1 and really image in detail with 4:1. If you can see what is doing the polishing, you'll also get a sense of what it will do to the glass surface. My point is you could image the surface to determine how this mystery white looks like, and then also image what the grit looks like, and what the effect of polishing is...just by taking some macro shots.

Click to expand...

I've had a look at the WC under a microscope, it has a smooth, very fine granular appearance, but as the material is glass it is viewed with both transmitted and reflected light which makes it difficult to see detail. I don't think macro shots wil improve on this.

From angle and without light ahead it can look like a solid.

Bosun Higgs · Sep 13, 2022

The Cerium Oxide powder finally arrived. I decided to first try polishing an element from the large triplet I had, this was a 17" f4 lens from an Epidiascope...... you can guess why I am interested in this lens type ;O)

Hand polishing turned out to be too slow to be practicable (no effect after an hour) so I used a polishing wheel with the Ce02. This did remove the White Curse (WC), and at a rate that was very controllable. I found that the WC layer itself could be used as a "guide coat". Polish for 5 min. wash and clean the lens and check for WC, I used a sharpie on the reverse surface to highlight the worst areas so I could target the polishing, then repeat.

After and before. The speckling on the polished lens is magnified gubbins on the background.

The picture shows one element before polishing and the other after. The clear element was actually the worst originally, all of its surface showed the WC in spades. Sorry that this shot is not ideal, but it gives you the idea.

I can still make out faint traces of the WC, especially at the edges as these are hard to polish with a wheel, but I don't want to risk overdoing it. The nice bit is that the outer face still retains its coating.

I was so impressed by the results that I thought I would try to restore a more immediately desirable lens, a Benoist Berthiot Super Cinestar 105mm f1.8.

Whilst not full-on WC, the coating on the front element was very degraded, producing a matte surface, the lens was unusable as the image was filled with foggy veiling flare and any bright specular sources resulted in a total whiteout.

Cinestars give some of the best "bubble" bokehs in my collection and this was the only Super Cinestar I had, so I was really cautious with the polishing on this one.

The lens is now usable, flare and glow have reduced hugely, there is still a little, but it is difficult to work out if this is due to residual WC, or just because the front surface no longer has a coating. Also this is a six element Double Gauss derivative, so I will have to check all of the other elements carefully for any coating degradation as well.

Despite the polishing the lens is sharp, and I'm looking forward to finding out what it can do bokeh-wise. I've included a few post-polish Cinestar test shots, it was very blowy, so getting optimum focus was difficult with the tiny DOF, but it definitely looks like a bubbler.

Nice smooth background bokeh.

Missed best focus here, but it's defintely a bubbler!

No veiling flare and no foggy speculars!

Pixel level crop from the above shot, sharp enough for me! There is a touch of glow here, but this is a wide-open repolished 100mm f1.8 after all!

None of these shots would have been possible pre-polish.

Given that the Cinestar was unusable, I'd say this was a huge success, but I'll have to wait until I've polished the second element from the big triplet to test that beast.

Well chuffed!

fferreres · Sep 14, 2022

Bosun Higgs said:
The Cerium Oxide powder finally arrived. I decided to first try polishing an element from the large triplet I had, this was a 17" f4 lens from an Epidiascope...... you can guess why I am interested in this lens type ;O)

Hand polishing turned out to be too slow to be practicable (no effect after an hour) so I used a polishing wheel with the Ce02. This did remove the White Curse (WC), and at a rate that was very controllable. I found that the WC layer itself could be used as a "guide coat". Polish for 5 min. wash and clean the lens and check for WC, I used a sharpie on the reverse surface to highlight the worst areas so I could target the polishing, then repeat.

After and before. The speckling on the polished lens is magnified gubbins on the background.

The picture shows one element before polishing and the other after. The clear element was actually the worst originally, all of its surface showed the WC in spades. Sorry that this shot is not ideal, but it gives you the idea.

I can still make out faint traces of the WC, especially at the edges as these are hard to polish with a wheel, but I don't want to risk overdoing it. The nice bit is that the outer face still retains its coating.

I was so impressed by the results that I thought I would try to restore a more immediately desirable lens, a Benoist Berthiot Super Cinestar 105mm f1.8.

Whilst not full-on WC, the coating on the front element was very degraded, producing a matte surface, the lens was unusable as the image was filled with foggy veiling flare and any bright specular sources resulted in a total whiteout.

Cinestars give some of the best "bubble" bokehs in my collection and this was the only Super Cinestar I had, so I was really cautious with the polishing on this one.

The lens is now usable, flare and glow have reduced hugely, there is still a little, but it is difficult to work out if this is due to residual WC, or just because the front surface no longer has a coating. Also this is a six element Double Gauss derivative, so I will have to check all of the other elements carefully for any coating degradation as well.

Despite the polishing the lens is sharp, and I'm looking forward to finding out what it can do bokeh-wise. I've included a few post-polish Cinestar test shots, it was very blowy, so getting optimum focus was difficult with the tiny DOF, but it definitely looks like a bubbler.

Nice smooth background bokeh.

Missed best focus here, but it's defintely a bubbler!

No veiling flare and no foggy speculars!

Pixel level crop from the above shot, sharp enough for me! There is a touch of glow here, but this is a wide-open repolished 100mm f1.8 after all!

None of these shots would have been possible pre-polish.

Given that the Cinestar was unusable, I'd say this was a huge success, but I'll have to wait until I've polished the second element from the big triplet to test that beast.

Well chuffed!

This is awesome.

The images look fantastic. The stereokeh photos are amazing. I still wonder if I could do that without careful practice, worry would change the angle of framing but you do it right every time.

How did you go about polishing (tool and part). Would be great to know the compound brand.

Congrats!

Bosun Higgs · Sep 14, 2022

fferreres said:
How did you go about polishing (tool and part). Would be great to know the compound brand.

Congrats!

The compound was pink Cerium Oxide 3.5um unbranded from a UK supplier. It is recommended to use a felt polishing mop, but I used a 75mm calico wheel.

You make a paste with the powder and water and dress the wheel with it, then apply to the glass. I used a hand held drill and kept moving the wheel over the glass constantly, with constant angle change.

You have to rewet and redress the wheel as you use it, you can tell when the CeO2 film starts getting sparse or patchy.

As the white degradation layer was easily visible I just kept cleaning the glass and checking, I marked the back of the element with a sharpie to indicate the areas to treat.

I was expecting that the polishing would ruin the sharpness on the Cinestar, but many of my untouched projection lenses are less sharp than the polished Cinestar.

I was so downhearted when I bought the lens, I have several Cinestars and Neo Cinestars, but I'd never seen a Super up for sale, so when it arrived and I discovered that the degraded coating made it unusable, I was gutted. Now and then I would see the lens in my collection and I would always feel pangs of regret for the poor thing, now it's restored I get a warm glow instead.

It's a bit like the feeling you get when you finally manage to convert a really difficult lens....... just turned up to eleven!

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