Super Multi-Coated Takumar 50/1.4 Yellowing

[RLangham]

So I just bought one of these well-regarded Takumar 50/1.4 lenses (with the linkage for wide-open metering on the later Spotmatics, but without the rubber grip), and it's a very nice lens in terms of build and in terms of versatility--image quality seems very good from f/16 to f/1.4, great close up performance, great performance at infinity.

But it does show pronounced yellowing, which is visible on the ground glass when looking through the lens. I haven't developed enough film from this lens to be able to tell what the average effect on the final image is. I almost want to expose it to either sunlight or my UV lamp to try and reduce the yellowing (said to work well), but I worry about losing the vintage feel of the lens.

So photographers who have shot with both yellowed and unyellowed versions of this lens, or who have any experience with the UV deyellowing process, what's your advice? Deyellow or not?

 

[Donald Qualls]

In the Super Takumar 50/1.4 version I have, the yellowing is supposed to be due to the effects of beta radiation from the thorium doping in one of the elements (near the rear of the lens -- yes, the lens is radioactive, and no, I've never seen evidence of it fogging film, but then beta has very little penetration -- but you probably shouldn't use it as a loupe on any sort of routine basis).

In my own experience, I was able to clear most of the yellowing by wrapping the rear end of the lens in aluminum foil and putting it on a windowsill in intermittent direct sun for, as I recall, a few weeks. The UV apparently undoes whatever atomic-level damage to the glass causes the yellowing -- or maybe it's just the visible light. I don't know, but it worked pretty well for me. Treatment may need to be repeated every couple decades, of course, but there will come a time when the loss of thorium from that element will affect the corrections in the lens enough that they'll all lose some optical quality. BTW, the same situation applies to some Aero Ektar lenses, which have a lanthanum glass, as well as Lanthar and Apo-Lanthar lenses.

The yellowing has little effect on B&W film in any case -- just acts like a rather weak yellow filter, much less than 2x filter factor -- and it will correct out readily on color negative films. If you shoot slides and have a very sharp eye for color, you might want to consider doing so with another lens.

 

[RLangham]

In the Super Takumar 50/1.4 version I have, the yellowing is supposed to be due to the effects of beta radiation from the thorium doping in one of the elements (near the rear of the lens -- yes, the lens is radioactive, and no, I've never seen evidence of it fogging film, but then beta has very little penetration -- but you probably shouldn't use it as a loupe on any sort of routine basis).

In my own experience, I was able to clear most of the yellowing by wrapping the rear end of the lens in aluminum foil and putting it on a windowsill in intermittent direct sun for, as I recall, a few weeks. The UV apparently undoes whatever atomic-level damage to the glass causes the yellowing -- or maybe it's just the visible light. I don't know, but it worked pretty well for me. Treatment may need to be repeated every couple decades, of course, but there will come a time when the loss of thorium from that element will affect the corrections in the lens enough that they'll all lose some optical quality. BTW, the same situation applies to some Aero Ektar lenses, which have a lanthanum glass, as well as Lanthar and Apo-Lanthar lenses.

The yellowing has little effect on B&W film in any case -- just acts like a rather weak yellow filter, much less than 2x filter factor -- and it will correct out readily on color negative films. If you shoot slides and have a very sharp eye for color, you might want to consider doing so with another lens.

Ah, so it takes weeks... well, thanks, Donald. I don't know that I want to take the lens out of commission that long right now. Maybe later.

I guess what I'm seeing on these negatives is that the scanner takes out the yellow tint automatically. This is odd and I didn't expect it, because this scanner readily keeps in the slight pink-red tint of some UV filters. I'm not set up for color enlargements so I don't imagine the yellowing will be a problem right now.

 

[MattKing]

The effect of the sunlight is cumulative - leave it on the windowsill when you aren't using it.
And the improvement in colour will reduce your post-processing times.

 

[Wallendo]

Window glass filters out some, but not all UV light If you have a UV light at your disposal, you could try exposing your lens to direct UV light overnight for a few weeks. I am not aware of any evidence that exposure needs to be continuous for long periods to be effective. You could still use the lens during the day.

 

[Donald Qualls]

Given it's an SLR lens, so will stay wide open when on the camera, you could also just park the camera under a UV light when not in use. Leave the cap off. The UV won't hurt the leather on the camera in the time it takes the lens to improve.

EDIT: Woops, I typed beta above, and natural thorium decay is an alpha emitter -- beta is more penetrating (electrons or positrons vs. the helium nuclei of alpha).

 

[JPD]

I almost want to expose it to either sunlight or my UV lamp to try and reduce the yellowing (said to work well), but I worry about losing the vintage feel of the lens.

"De-yellow" it if you want the real vintage feel. The glass wasn't meant to be yellow, so by removing it you will restore it to how it was meant to be when it was made back in the day. If you keep the yellow tint, you get the "year 2020 detoriated glass" feel. It's a defect, just like hardened grease can glue a focus ring in place.

 

[Scott Micciche]

I found using a reptile light (LED, no heat), reduced the yellowing of my 105mm f/2.4 by 95% in just 24 hours. I did remove the element so other elements would not get in the way. These thorium lenses have a real nice quality to them, it you shoot color film, it is worth de-yellowing.

 

[RLangham]

I found using a reptile light (LED, no heat), reduced the yellowing of my 105mm f/2.4 by 95% in just 24 hours. I did remove the element so other elements would not get in the way. These thorium lenses have a real nice quality to them, it you shoot color film, it is worth de-yellowing.

I have a fluorescent blacklight tube I bought for doing cyanotypes. Unfortunately I am definitely not competent to disassemble a lens. I tried once...

 

[Scott Micciche]

I have a fluorescent blacklight tube I bought for doing cyanotypes. Unfortunately I am definitely not competent to disassemble a lens. I tried once...

It will work just fine without disassembly. I wanted to clean the elements anyway so I had it apart. Blacklight tube might take a bit longer, but it will work and is also cold. You do not want to warm the lens. Take some photos each day to see how it reduces.

 

[JPD]

It will work just fine without disassembly. I wanted to clean the elements anyway so I had it apart. Blacklight tube might take a bit longer, but it will work and is also cold. You do not want to warm the lens. Take some photos each day to see how it reduces.

Maybe it would speed things up if the lens stands on a mirror so the UV light that passes through the glass reflects back up.

 

[Scott Micciche]

Maybe it would speed things up if the lens stands on a mirror so the UV light that passes through the glass reflects back up.

Yes, I've read this can help. Some use aluminum foil as a reflector.

 

[Steve Roberts]

My 50mm f1.4 Super-Takumar took about a week of daily exposure to strong sunlight to get shot of the yellowing, though I suppose much depends on the extent of the yellowing when you start. I wrapped it in aluminium foil, front element left uncovered, and left it on a window sill, turning it occasionally to make the most of the sunshine as the sun travelled across the sky. OK, I know the sun doesn't actually move anywhere but I'm sure you get my meaning! The hardest part was finding a succession of sunny days in SW England!
Steve

 

[Patrick Robert James]

I have the ends of UV string lights that I built a UV exposure unit out of. And I have a Takumar with the yellowing. I was going to stick it in the sun, but I think I might just use the few UVs I have left. Worth a shot. Let's see how long it takes.

 

[AgX]

There will come a time when the loss of thorium from that element will affect the corrections in the lens enough that they'll all lose some optical quality.

Thorium has a half-life of 14 billion years.

I typed beta above, and natural thorium decay is an alpha emitter -- beta is more penetrating (electrons or positrons vs. the helium nuclei of alpha).

Things are more complicated. What you refer to is the emission of the decay of Thorium to just the next step in the decay ranking. But within in this ranking are steps with have Beta- and Gamma-radiation.
One thus has to consider a mixture of radiations from various steps, all taking place simultaniously.

 

[Bikerider]

I have a Minolta 28/F2.8 MC lens that is known to have thorium in the the glass. It doesn't bother me because it renders B&W beautifully without having to use a yellow filter to bring out the clouds. I also have 'normal' 28mm if I want to use the lens for colour neg work.
I suppose I could dial out the yellow in the printing stage if I wanted to, but really I cannot be bothered.

 

[reddesert]

Radiation damage to glass causes defects in the structure of the glass by exciting electrons to move into different places in the glass (these would be defects in the crystalline structure, but glass is amorphous and not a crystal). These defects cause absorption that looks yellow or brown for mild damage. This damage is typically called "browning," so that helps find references to the subject. Heat and UV light can reverse mild damage, presumably by freeing the electrons to move out of trapped defects. For example
http://birns.com/uploads/file/Radiation-induced Discoloration.pdf
https://www.steris-ast.com/techtip/radiation-processing-glass-coloration-discoloration/

Decay of thorium produces other radionuclides including radium and radon. There are some alpha and gamma emissions in this process but the gammas are pretty weak. While it's true that gammas are more penetrating, the primary concern for this kind of low-level radiation is to avoid ingesting the radionuclide, no matter what particle is being emitted. I wouldn't use it as a loupe for long periods of time, but just handling it is NBD. Ideally, such lenses should be kept intact. I mention this only because there was once a thread where someone wanted to destroy the lens for "safety" reasons, which is exactly the wrong thing to do; one doesn't want to generate a lot of thorium dust.
https://semspub.epa.gov/work/HQ/175255.pdf

 

[Donald Qualls]

Thorium has a half-life of 14 billion years.

Well, that's a relief. My lens will still be legendary when my civilization is no longer even mythical, at least as long as it gets enough UV to keep the glass tint in check.

Radiation damage to glass causes defects in the structure of the glass by exciting electrons to move into different places in the glass (these would be defects in the crystalline structure, but glass is amorphous and not a crystal). These defects cause absorption that looks yellow or brown for mild damage. This damage is typically called "browning," so that helps find references to the subject. Heat and UV light can reverse mild damage, presumably by freeing the electrons to move out of trapped defects. For example
http://birns.com/uploads/file/Radiation-induced Discoloration.pdf
https://www.steris-ast.com/techtip/radiation-processing-glass-coloration-discoloration/

Decay of thorium produces other radionuclides including radium and radon. There are some alpha and gamma emissions in this process but the gammas are pretty weak. While it's true that gammas are more penetrating, the primary concern for this kind of low-level radiation is to avoid ingesting the radionuclide, no matter what particle is being emitted. I wouldn't use it as a loupe for long periods of time, but just handling it is NBD. Ideally, such lenses should be kept intact. I mention this only because there was once a thread where someone wanted to destroy the lens for "safety" reasons, which is exactly the wrong thing to do; one doesn't want to generate a lot of thorium dust.
https://semspub.epa.gov/work/HQ/175255.pdf

Yes, whatever thorium-2 is on the periodic chart is still radioactive. Everything that (atomically) heavy is radioactive, at some level. But you've elaborated my point -- the lens itself isn't a hazard. Dust form the thorium glass more so.

You're almost certainly at greater risks from natural radon, if you live in or near the Appalachians (especially if you have a basement). Just about the only thing with higher risk of lung cancer than smoking.

 

[RLangham]

The funny thing is, I also have the lanthanide-doped Industar 61L/D which hasn't yellowed at all, despite having a reputation for being more radioactive.

 

[Donald Qualls]

Browning may be codependent on other dopants in the glass -- that is, alpha radiation knocking stuff around may not have much effect on a glass that doesn't have, say, a high iron or magnesium content (just as examples, I don't think there's much of either in most optical glasses because they make glass green). In other words, it may also matter what else is in the glass.

 

[AgX]

The funny thing is, I also have the lanthanide-doped Industar 61L/D which hasn't yellowed at all, despite having a reputation for being more radioactive.

Lanthanum is not radioactive.
Furthermore to my understanding there are no soviet consumer lenses that are radioactive.

(I would like to add, that I got radioactive lenses, and I know that people do methodically wrong metering.)

 

[Donald Qualls]

I was probably conflating lanthanum with actinium -- next row down on the table. All of the actinides are radioactive, as I recall; that's the family that contains thorium, uranium, and plutonium.

 

[RLangham]

Lanthanum is not radioactive.
Furthermore to my understanding there are no soviet consumer lenses that are radioactive.

(I would like to add, that I got radioactive lenses, and I know that people do methodically wrong metering.)

Well, it seems you're right. But it also seems that this is a very popular misconception. A lot of websites repeat that the I-61L/D is radioactive, with no apparent source other than a common belief that lanthanum is unstable and therefore radioactive.

A bit of googling shows that there is an unstable natural isotope of lanthanum, but it decays so slowly as to be practically stable, and that all other unstable isotopes decay so quickly that they last only days to minutes when made synthetically, so, indeed, it would seem that lanthanum, while technically one of the elements that CAN be radioactive, in practice it never is to any meaningful degree.

 

[AgX]

Even a german radiation authority lists Lanthanum as non radioactive.

 

[RLangham]

Even a german radiation authority lists Lanthanum as non radioactive.

When something decays so slowly that it randomly emits a particle every few thousand years or something like that, I imagine there's no practical reason to consider it radioactive.