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04-10-2014, 12:51 PM   #1
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Curing of yellow color of old lens coatings by exposing the lens under UV light

Hello all,
I have read on the various foto forums, that the coloration of old lenses can be reduced by exposing the lens under strong UV for a few hours, this procedure somewhat increases the sharpness and the resolution of lenses. Is it true? My favorite Takumar 50/1.4 is significantly yellow. I have UV lamp in the lab. How long time is really required?

Can this procedure damage the lens coatings or accelerate senescence of my lens?

Is such a treatment applicable for Pentax 67 MC lenses?

Thank you in advance.


Last edited by podvalnyy; 04-10-2014 at 12:52 PM. Reason: orthography
04-10-2014, 02:53 PM   #2
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It is my understanding that the yellowing is not related to the coatings but to the slight degradation of the adheasive used to bind certain lens elements together. The ever so weak radiation produced by thorium-glass elements is the culprit.

None of my Takumars show this colouring to any noticeable degree, but from all that I have read, there seems to be consensus that the proper cure would be somewhat as described here:

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04-10-2014, 03:11 PM   #3
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The aluminum foil method works. I've used it on two lenses. It took two weeks or a bit more sitting on my south facing window sill, behind a single pane of dirty glass.
04-10-2014, 03:27 PM   #4
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Strong UV - good daylight should be enough, just make sure the lens isn't focused so it doesn't start a fire

04-10-2014, 07:48 PM   #5
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I used a stamp collector's UV bulb in a standard desk lamp fixture with a fan set up nearby to cool the bulb and it cleared up a heavily yellowed Super Tak 50mm in less than a day. I put a piece of foil in the rear cap to act as a reflector and speed up the process.

This test showed that some wavelengths seem to work better than others under medical lights; with a cheap black light bulb the spectrum is wide enough that it just works: https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/218383-yell...avelength.html
03-14-2017, 09:41 PM   #6
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Treating yellow lenses with LED UV grow lamps

Old thread, but I'm about to use a LED UV lamp to get rid of some tint in 2-3 lenses I have, including a 28, 35 and 50mm (...the ones you never uses, of course..).
Bought this from Amazon, as my apartment is located in a place where the sun never hits a single window...:


I hope it'll work, as I'll need this in the upcoming days for a shoot.
03-14-2017, 10:06 PM   #7
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While the Canadian Balsam used to cement lenses together can yellow, the yellow in Thorium Glass ("Radioactive") lenses is due to chemical changes in the chemicals in the glass itself.

So you can have yellow lenses that are not at all "radioactive".

As I recall I posted some links to some papers on the discoloration phenomenon of Thorium Glass and UV light in other threads.

Radioactive lenses
QuoteQuote:
However, the early glass formulas and contaminated chemicals used have produced lenses which
have "yellowed" with age (see posts). This yellowing is related to on-going chemical reactions in
the lens related to the chemistry of the early glass formulas used in high refractivity glasses. See
our lens color pages for tips on how to test your lenses for this thorium glass yellowish coloration.
QuoteQuote:
However, don't assume that all yellow lenses are necessarily radioactive. Many lenses turn yellow
due to aging of the Canada Balsam adhesives used to glue lenses together. Other lenses may have
coloration biases which make them slightly yellowish rather than clear, or other colors depending
on the glass. But if you have a lens using early specialty glasses (wide angles, fast lenses..) from
before the 1970s, you should consider checking your lens for radioactivity. Most school physics
labs can do this, as can local Civil Defense offices and police or fire emergency response teams.
Another important point here is that these contamination levels vary from batch to batch,
depending on the contamination in the sands used to make the impure chemicals being used. So
you have to test each lens to determine if it is radioactive, and how "hot" any given lens really is.
Tests of one lens from one batch won't apply to another lens made from another batch of
chemicals with different levels of radioactive contaminants. You have to test each lens to be sure.
03-15-2017, 02:58 AM   #8
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QuoteOriginally posted by Marc-Alexandre Quote
Old thread, but I'm about to use a LED UV lamp to get rid of some tint in 2-3 lenses I have, including a 28, 35 and 50mm (...the ones you never uses, of course..).Bought this from Amazon, as my apartment is located in a place where the sun never hits a single window...:

I'd recommend doing the de-yellowing in stages and testing the effect on the colour rendering as you go along. My personal feeling is that a lot of the reputation of the 50mm/1.4 comes from the warm romantic look given by slight yellowing.

When I first got my own slightly yellowed 50mm/1.4 I was blown away by the results it produced, so I decided to completely de-yellow it using an LED, in the belief that this would make it even better. But it didn't. After de-yellowing the magic of the colour rendering was gone, and what I ended up with was a lens that I liked a lot less than my much cheaper Super Takumar 55mm. So I sold the 50mm -- for more than I paid for it because the glass was now clear.

Of course this is just personal opinion, but it's an opinion based on experience. For me at least, the 55mm/1.8 produces much better colour rendering, contrast and sharpness than a fully de-yellowed 50mm/1.4.

04-09-2017, 05:02 PM   #9
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QuoteOriginally posted by Dartmoor Dave Quote
I'd recommend doing the de-yellowing in stages and testing the effect on the colour rendering as you go along. My personal feeling is that a lot of the reputation of the 50mm/1.4 comes from the warm romantic look given by slight yellowing.

When I first got my own slightly yellowed 50mm/1.4 I was blown away by the results it produced, so I decided to completely de-yellow it using an LED, in the belief that this would make it even better. But it didn't. After de-yellowing the magic of the colour rendering was gone, and what I ended up with was a lens that I liked a lot less than my much cheaper Super Takumar 55mm. So I sold the 50mm -- for more than I paid for it because the glass was now clear.

Of course this is just personal opinion, but it's an opinion based on experience. For me at least, the 55mm/1.8 produces much better colour rendering, contrast and sharpness than a fully de-yellowed 50mm/1.4.
Well that's an interesting turn of events.

Was just discussing this on another film forum. I've a Takumar 105/2.4 6x7 lens that I got an amazing price over a year ago. Here in WA state, even bright sunny days are often hazy and it was only marginally improved by 2 days of exposure (all the while trying to keep the direct rays angled properly). But I did find a good cheap fix (below).

Regarding the Thorium-yellowing in terms of coloration of photos, it really wont' make much of a difference on film, however I 85% cleared my VERY yellowed Tak 105/2.4 using this very inexpensive grow light. It took about a week - mine cleared to 50-60% the first two days or so and then it slowed down increasingly until finally I said "that's good enough".

I'd heard LED light wouldn't make enough output to do anything, but most incandescents generate significant heat (I tried one for a while but gave up as I was hesitant to leave it overnight or even unattended).. This light here that I used, being a cheap grow light seemed to prove the LED-won't-work theory wrong. The nice thing about it as well there is virtually no heat, unlike others, and the size and design made it so I could clip it to the bookshelf and bend the gooseneck down so the light was almost sitting on the lens. I did it with both caps off, turning the lens over every other day so to shine from both the front and back. For the first two days I did ONLY from the back since that's where the throrium element is. Turned out to be pretty easy.

Here's the light:
https://www.amazon.com/gp/product/B01ANSUVWA/ref=oh_aui_search_detailpage?ta...&ie=UTF8&psc=1

I don't have (of course) a before and after photo, but my 105 looked as yellowed (if not moreso) as the one in this Ebay link:
Pentax SMC TAKUMAR 105mm f/2.4 for 6x7 67 MF Lens From Japan Exc *0819 | eBay

And it now looks like this (still not perfect but hugely improved and good enough for me):
04-16-2017, 10:45 AM   #10
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Chickentender's photo of through-transmission made me wonder whether anyone has tried using the in-camera white balance calibration feature, using a grey card or something similar, to compensate for the radiation color shift. I can't imagine that the tint would be so pathological that this wouldn't work, though I don't have lenses of this type for verification. But all you'd have to do is store the calibration in one of the custom WB settings, and call it up afterward whenever you mount the lens. Of course, if you're in the habit of routinely customizing WB to the scene, you shouldn't have a problem in the first place.

I can only think of 2 disdvantages:
1. The radiation tint might reduce the T-stop light transmission significantly, so getting rid of it might "speed up" the lens a bit..
2. You wouldn't have a "bleached" vintage lens to sell for a premium on ebay!
05-31-2017, 09:53 PM   #11
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QuoteOriginally posted by Dartmoor Dave Quote
For me at least, the 55mm/1.8 produces much better colour rendering, contrast and sharpness than a fully de-yellowed 50mm/1.4.
For me, it was about consistency of the lenses through the kit. Having a lens that has yellow tint in it isn't good for me (video-wise) so I "had" to perform the UV treatment on some. At least now, they're equally (mostly) not yellow tinted. So for video, it's perfect as I can switch lenses and I don't have to color correct each shot in post after that!
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06-02-2017, 07:39 AM   #12
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Interesting thread, my Super Tak 50mm f1.4 has heavy yellowed glass.
Anyway I´m not yet decided to "fix" it, I think the yellow tint adds a lot of character to the lens.
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06-02-2017, 07:51 AM   #13
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QuoteOriginally posted by Cabessius Quote
Interesting thread, my Super Tak 50mm f1.4 has heavy yellowed glass.
Anyway I´m not yet decided to "fix" it, I think the yellow tint adds a lot of character to the lens.
Would work well with b&w film, no need for a yellow filter!

Phil.
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