How can I safely remove lens coatings ?

[John Salim]

I have some old ( non-camera ) eyepieces, all of which were badly 'cleaned' over the years leaving deep looking lines on the coating ( not glass scratches ) which I'd like to clean off back to the glass.
If there a safe way of removing these coatings ?
They are from the mid 1980's and 'bloomed' blue ( if that helps ? )

Many thanks,
John S

 

[bernard_L]

Cerium oxide. Look for amateur telescope construction.

 

[koraks]

I have some old ( non-camera ) eyepieces, all of which were badly 'cleaned' over the years leaving deep looking lines on the coating ( not glass scratches ) which I'd like to clean off back to the glass.

Why would you want to remove the coatings? It'll only make matters worse.

 

[bernard_L]

Why would you want to remove the coatings? It'll only make matters worse.

Indeed ! (I was just replying to the OP's question)

Say, the coated surface has a 1% reflectivity (vintage coating) and the uncoated surface 3% (just numbers for a concrete argument)
So, staring at the lens in reflection, the uncoated spots reflect 3x as much light as the coated ones; +200%! Horrible!! But the lens operates in transmission. Coated surface transmits 99%, uncoated 97%. Ratio 97/99~0.98. Just a 2% decrease in transmitted light.
Yes, there is also flare to consider. But why not enjoy your lens as-is?

 

[koraks]

Say, the coated surface has a 1% reflectivity [...]

That's exactly what I was thinking of; you used this example the other day and it came to mind when I saw this question. The practical impact of the scratches will be negligible. Removing the entire coating, however, will make a difference in reflections/glare/contrast. Not to mention that there's the risk of abrasions to the lens elements, which will exacerbate any problems.

 

[Graham06]

I've been through this. I bought a big 600mm barrel lens from a bad ebay seller and the coating was badly damaged. You'd think that it would just be a case of having a mostly good coating, but the edges of the scratch perturb the surface a little, and the image looked like I had a soft focus filter installed.

I removed the coating and the image was much better. I can't remember what I used though. I have cerium oxide and pitch for a lapping tool, but I am not sure that I had to go to that effort. I think I tried some other solvent first. Perhaps I tried acetone. I have quite a few solvents, so perhaps it was something else (e.g. the stuff you use to remove nail polish from plastic nails)

If you go this route, you should remove the lens element from the rest of the assembly, and then still be careful if the lens is cemented to another element. Also, check your optics first. I think there are many types of coating and the story the other posters are telling might be true for your coating.

 

[John Salim]

Why would you want to remove the coatings? It'll only make matters worse.

These eyepiece lenses are on my spare IR darkroom goggles, and the image looks very flarey.
If I could remove the coating, I'm sure it'll improve them no end.

Cerium oxide sounds promising - thank you Bernard

John S

 

[Sirius Glass]

Why would you want to remove the coatings? It'll only make matters worse.

Exactly my thoughts. Quit while you are ahead!

 

[Rick A]

I find 40 grit paper on my random orbit sander works wonders. Or I don't pay attention to them and just live with the marks.

 

[bernard_L]

Why would you want to remove the coatings? It'll only make matters worse.

Exactly my thoughts. Quit while you are ahead!

Now that I've read the extra information from the OP:

These eyepiece lenses are on my spare IR darkroom goggles, and the image looks very flarey.

I understand he does not experience just "cleaning marks" on the coating, but that the coating has deteriorated as a whole, becoming "milky".
I've seen this a few times:

• On the front surface of a Contaflex Tessar:
• On an inner surface of a Pentacon auto 2.8/135 MC (aka bokeh king)
• On the mirror train of the VLTI (interferometer of ESO's Very Large Telescope at Paranal, Chile)

In that case, removing --properly-- the coating may improve the performance of the lens.

 

[John Salim]

.... in an ideal world, it would be good if there was a was to safely dissolve the coating ( rather than using physical force ).

JS

 

[250swb]

Another way to remove coatings is to have somebody else do it for you, these are a Nikon 50mm from a newspaper photography department supply cupboard, they didn't bother with lens caps

 

[baachitraka]

Don't bother and don't touch it. Just use it and prints or scans may never notice anything.

 

[koraks]

Just use it and prints or scans may never notice anything.

I have some old ( non-camera ) eyepieces

These eyepiece lenses are on my spare IR darkroom goggles

Scans and prints will surely not notice anything from lenses that aren't in the optical path to begin with.

 

[John Salim]

If you'd like to see the problem ....

 

[Ian Grant]

Why would you want to remove the coatings? It'll only make matters worse.

A subject U discussed with a photographer on Saturday, he has a Kodak Ekttar 127mm and the coating is damaged. It's damage in one area, with B&W images it makes no visible difference, but with C41 negatives he says there's a patchy colour balance change.

I have some old ( non-camera ) eyepieces, all of which were badly 'cleaned' over the years leaving deep looking lines on the coating ( not glass scratches ) which I'd like to clean off back to the glass.
If there a safe way of removing these coatings ?
They are from the mid 1980's and 'bloomed' blue ( if that helps ? )

Many thanks,
John S

By the mid 1980s most lenses had excellent hard coatings, the big change was multiple coatings (as in more than one layer/type) in the early to mid mid 1950s to ensure good colour balance, Early Carl Zeiss coating introduced in 1938 was excellent and hard wearing but gave a slightly cold blueness. I have/had a 1953/4 CZJ T coated 150mm f4.5 Tessar, and it is a noticeable blue tinge.. Lenses like the CZJ 50mm Pancolar, the Colour Skopar, etc, had cotings better optimised for colour work.

Kodak's Blooming was probably amongst the softest coating. I have had quite a few Kodak 203mm f7.7 Ektar lenses, most UK made, and you can see changes in the coatings.
The poorest are the early ones in Kodak Epsilon shutters, these are in catalogues as "Bloomed", later one's in Prontor #0 SVS shutters have excellent coatings, as do the last in #0 Synchro Compurs which have better colour corrected coatings. I only have one Eastman Kodak version, which is one of the last mage, and in a Graphic Syncro Compur #1.

If you'd like to see the problem ....

That element would need removing a re-polishing by someone experienced.

Ian