[K David]

Can the OP share some photos of the lens' yellowing?

[glasbak]

Then the K-mount ones aren't radioactive?

Both my 1.4/50 K mount lenses are as radioactive as mu SMC-taks.

Incorrect, It is actually the Canada ba salm adhesive between the lens elements that is being affected by the radiation. If the glass itself was being affected by radioactivity the yellowing would be homogenous throughout the glass. It would be interesting to see if modern UV curing optical adhesives offer superior resistance to this effect. The problem is that it would take a considerable amount of time to test this hypothesis.


Rear cell from a M42 Super-Takumar 50mm f/1.4 showing yellowing caused by Thoriated lens element - lens S/N 4124605

Well, as example you show a yellowed single element, which is not cemented, so it is the glass which yellowed .

[Digitalis]

as example you show a yellowed single element, which is not cemented, so it is the glass which yellowed

Actually, the elements i'm holding in that photograph which have been removed from the rear cell of a Super-Takumar 50mm f/1.4. (In the 7 element Takumar these are the 4 and 5th elements*) these two elements are a cemented doublet, and it is plain to see that the cement itself has discoloured - not the glass.

my understanding is that they had got away from the radioactive elements during the building of the last few years of the m42 lenses. Asahi cataloged both m42 lenses and K-mount lenses simultaneously for a few years though.

I have a theory about that Blue - perhaps the adhesive Pentax has been using to cement the lenses had been changed, perhaps a more modern adhesive with the correct optical properties that doesn't discolour like Canada basalm. In which case the only way to prove beyond a shadow of a doubt that you have a radioactive lens would be by using a Geiger counter.


*counting from the front- to-back, the thoriated lens elements are in basically the same position in the 8 element version though they are a bit harder to remove from the lens.

[glasbak]

Actually, the elements i'm holding in that photograph which have been removed from the rear cell of a Super-Takumar 50mm f/1.4. (In the 7 element Takumar these are the 4 and 5th elements*) these two elements are a cemented doublet, and it is plain to see that the cement itself has discoloured - not the glass.

Well, I can only advice that you read this page, and do not stop reading at the table, but also check the part where I disected a radioactive 1.4/50mm takumar.

[sam-joseph]

Sorry to jump into the middle of your argument discussion, but I have a Super-Tak 55mm f1.8 which is very yellow. I wrapped it in foil and put it on a windowsill inside the house for a couple of days with no change in the yellow hue. One of my learned colleagues informed me that window glass filters UV, which means the lens was never going to clear. Since then I've given it about 6 hours in the sun outside, and there is an obvious improvement. Thanks for listening, now back to you.

Regards

[Digitalis]

To quote your findings "It is the glass itself which turns yellow, I cannot rule out that the cement not turns yellow also, but find it unlikely."

Crystals have a different structure from glass - Glass is classified as an amorphous non-crystalline solid - crystal is of course a structured rigid lattice that is physically stable and at the molecular level - largely homogenous. The leaded borosilicate glass types found within these lenses have drastically different chemical and mechanical properties than your simplified "smoked quartz" hypothesis is based upon.

The chemistry present with a crystal coupled with the structured lattice crystals have makes it easy for "colour holes" to be formed. But in a glass, especially like that used in commercial lenses* with such varied chemistry and unordered structure makes this highly unlikely - your theory does have merit for originality however, I'm a skeptic.

The reason for this is that naturally derived resins when refined and used as adhesives have a known history of discolouration and disintegration in the presence of radiation. It is also ignoring the fact that lens makers go to considerable lengths to make sure their lens is as clear and neutral as possible and has the required refractive as well as physical properties. Lens makers also make sure the chemicals used in the glass aren't going to react with each other.

*that also contain other elements : like Lanthanum, Flouride, Magnesium,Titanium, Aluminium,Tantalum,Zirconum,Potassium, Sodium,Phosphor, Lead and Boron.

[glasbak]

To quote your findings "It is the glass itself which turns yellow, I cannot rule out that the cement not turns yellow also, but find it unlikely."

The leaded borosilicate glass types found within these lenses have drastically different chemical and mechanical properties than your simplified "smoked quartz" hypothesis is based upon.

It is not my theory, it is the description I found on the internet that did fit the best with my findings.
Feel free to come with a better one, but the canada balsam theory does not fall in that catagory, having yellowed single element non cemented lenses rules that out as the only source of yellowing.

And if you want much more hits on this subject, search for 'browning', and not yellowing.
Then you will find several companies offering special glass for radioactive environments which do not turn brown.
So it is not only crystals.

[Blue]

Here is an additional read, but old, on thorium in general.

Radioactive lenses