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UltravioletPhotography

Should I remove coatings from this lens


BruceG

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Recently I bought a broken Rollei 35LED and harvested the lens on it, Triotar 40/3.5, which is a triplet. Upon some immediate tests, I found this lens to be half a stop slower than EL-NIKKOR 80/5.6 (metal version) when both are at f5.6, and at f3.5 Triotar is one stop faster than EL-NIKKOR 80/5.6 at f5.6.

 

The thing is this lens looks like multi-coated because of the magenta color reflected from the elements, I used diamond pasted and successfully removed the coatings on the front and back of the front element, and front side of the second element, basically 3 surfaces out of all 6. Now the polished elements reflect white instead of magenta. Diamond paste is extremely good at the job, I only found out about it recently. See the picture below, there are two bright white reflections in front, one yellow behind which is the layer on the front of the second element I didn't remove completely, and three purple/magenta reflection behind, before the polish, all six look the same purple/magenta.

 

I did the conparison with EL-NIKKOR 80/5.6 again, but the difference in exposure did not change at all, as well as the colors of the photo (Triotar renders slightly more purpleish than EL-NIKKOR). Also noted is that the contrast of the lens is not affected too after the removal, both in UV and VIS.

Here is the test photo after the coating removal

 

Shot on A7S with AndreaU MKII filter, same in camera white balance setting

Triotar at f3.5, ISO1000 1/80s EL-NIKKOR at f5.6, ISO2000 1/80s

 

My question here, is it because some types of coating do not interfere with UV, or I have to remove all layers to see some effect?

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Andy Perrin
I would think disturbing any of them would have an effect if the coating was interfering with the UV in the first place. The coating thicknesses have to be very precise to get the right kind of constructive/destructive interference effects. Since removal had no effect, it is possible the coatings were for some other wavelengths.
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I would think disturbing any of them would have an effect if the coating was interfering with the UV in the first place. The coating thicknesses have to be very precise to get the right kind of constructive/destructive interference effects. Since removal had no effect, it is possible the coatings were for some other wavelengths.

Looks like so, however I find it hard to believe the coating actually is not doing anything to UV. Since so many dedicated UV lens are not coated, it only suggest minimal coating is best for UV transmission.
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It is also most likely that the attenuation by the coatings is stronger at shorter wavelengths.

It might be that your modification is not affecting the light situation in these motives enough to be visible after white balance.

I might also be the types of glass used in the lens that causes the UV-falloff.

 

Have you tried the pin-hole method? It is described elsewhere in the forum.

 

What kind of diamond paste did you use and how did you polish?

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Andy Perrin
Minimum coating is best for UV lenses in general. But I think you proved that in this case it was not the issue.
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Coatings are a crap shoot. Some seem to affect UV bandpass, whereas many do not. Ultraviolet bandpass may not have been much of a priority for lenses primarily designed for b&w film photography, as the filters used for that are themselves strong UV blockers. Older lenses come from an era when color film was,still more exception than rule.
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I just painstakingly removed all coating, the result is a total disappointment, no inprovement in UV at all, and the picture quality became a lot worse, internal reflection makes the picture become so hazy. When I removed the first three layers in front, I don't see a big difference, but this time the change was drastic, this lens with good contrast just became a soft focus lens.

 

What I learnt is that the coating on the rear elements are so much more important than the ones in front, next time I will just to a trial on removing the coating from the frontal element, if no improvement then I would not remove any further.

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It is also most likely that the attenuation by the coatings is stronger at shorter wavelengths.

It might be that your modification is not affecting the light situation in these motives enough to be visible after white balance.

I might also be the types of glass used in the lens that causes the UV-falloff.

 

Have you tried the pin-hole method? It is described elsewhere in the forum.

 

What kind of diamond paste did you use and how did you polish?

I have used the pinhole method so far only as a rough gadge on a lens' performance, the actuall performance sometimes can deviate from the estimation by as much as 1 stop!

 

As for my method of removing lens coating, I used a grade W2.5 diamond paste, interestingly, when I used a sponge to polish, the coating is only removed partially, a yellow layer alweys remains, when I switch to using a cotton, then that yellow layer is removed too. I have used the same method before to polish some badly scratched lens elements too, the results have good.

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The haze observed after removing coatings is likely due to a patina of microscratches on the glass, not due to the absence of coatings as such. Another reason not to try this, IMO--I have heard this issue reported before. I do not see any reason why coatings on the rear would be more or less likely to cause trouble than coatings anywhere else. Coated lenses also have internal coatings between elements that may equal or outnumber those on exterior surfaces.
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The haze occured right after I polished all the elements an put them together, however after two hours, I did a test shot again and it was gone. I am very sure I did not clean the lens in between, because I already cleaned it upon assembly. Here is how the haze looked like.

A few day later, I repeated the compasrion test under the same weather, same time of the day as the last one.

 

Triotar at f3.5, ISO1000 1/60s EL-NIKKOR at f5.6, ISO2000 1/60s

I would say while the difference in exposure is not obvious, the lens did lose some contrast.

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The first (hazy) image in post#10 looks to have been taken a very different filtration, not fully UV, with a lot of IR-contamination.

See the trees, number at the house close and the blue on the houses in the background.

 

The haze can be due to internal reflections in the lens of Vis and IR.

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As for my method of removing lens coating, I used a grade W2.5 diamond paste, interestingly, when I used a sponge to polish, the coating is only removed partially, a yellow layer always remains, when I switch to using a cotton, then that yellow layer is removed too. I have used the same method before to polish some badly scratched lens elements too, the results have good.

 

Can you please be a bit more specific (picture?) with what you mean with "a cotton"?

I might like to try removing some AR-coating myself.

It is also interesting to know a bit more about your diamond paste.

 

One of my Kuri-clones, an early Soligor, is a mess in much of the iris mechanism and I have given up trying to make it work on a camera.

I'm playing with the idea to measure the transmission with my spectrometer fist, and then remove all coatings before remeasuring.

That would be an interesting project for a rainy day.

If I get to doing that I will post the results here on the forum.

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So do you think it was something that evaporated? A solvent, or moisture?

My room is not air conditioned and weather did not change as far as I can feel, I totally have no clue.
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Can you please be a bit more specific (picture?) with what you mean with "a cotton"?

I might like to try removing some AR-coating myself.

It is also interesting to know a bit more about your diamond paste.

 

One of my Kuri-clones, an early Soligor, is a mess in much of the iris mechanism and I have given up trying to make it work on a camera.

I'm playing with the idea to measure the transmission with my spectrometer fist, and then remove all coatings before remeasuring.

That would be an interesting project for a rainy day.

If I get to doing that I will post the results here on the forum.

I took out the cotton from some cotton swab and just grab on my fingers to do the polishing. A much gentler way is to use a sponge, as its soft-ness absorbs the force and distribute it evenly, however, the last layer of coating on my lens which is yellow is never removed when I used the sponge.
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Bruce, the diamond polish might not be good enough. From metallographic work I know, you might need some further polishing with something even finer. Also your picture from the polished lens seems to show a not really "mirror" polished surface (I do not know the right word for the perfect surface).

 

I hope this link works and gives a clue:

 

https://www.buehler....suspensions.php

 

The finer the polishing step, the more time is needed for it.

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