[Blue]

The 49mm version certainly does.

Color of the coating is NOT the sole indicator of thorium in the coatings. Geiger counter can be used in conjunction with several non-thoriated lenses.

[Blue]

Huh? It was my understanding that most of the ST 50/1.4 (particularly later copies) were thoriated along with all S-M-C/SMC Takumar 50/1.4.

That being said, it had been my understanding that Pentax ceased production of lenses with thoriated glass with the K-series. The link in the comment above seems to indicate that this is not correct.

For general interest, here is a link that I have posted in the past showing relative radioactivity (both counts and gamma intensity) for multiple lenses from the late 1960s through the mid-1970s.

Radioactive lenses -- group shot | Flickr - Photo Sharing!


Steve

(...only owns one "hot" lens, an Auto-Rikenon 55/1.4 and yes, it is yellowed...)

Steve, what part of "not many other" Taks was confusing? Did you look at the lens in the original post and overlook that it is a K 50/1.4?

Edit: The radiation numbers and serial numbers are interesting on that chart. It is also interesting that a 20mm lens came up with a very low reading. He drew an conclusion int there somewhere that the 55/1.8 and 55/2 may have different optical formalae due to the radiation levels and that part is a stretch since the biggest difference is the aperture diaphragm.

[boriscleto]

The 49mm version certainly does.

I have a late production 67mm thread version. It has not yellowed.

[monochrome]

IMHO we're arguing over an unproveable. IIRC Pentax has long been known for poor recordkeeping - and what there was is likely in the Hoya dumpster.

I suggest the OP should just try the usual foil and windowsill cure and report what happens.

[V10]

Color of the coating is NOT the sole indicator of thorium in the coatings. Geiger counter can be used in conjunction with several non-thoriated lenses.

The coating is not the source of the yellowing. I had to sun bleach the lens along with my 1.4 and the yellowing is no longer as severe as in the shot posted.

[glasbak]

True, but there is no evidence of any SMC K ever having thorium. It wasn't put in manyl TAK focal lengths.

How much proof do you need ?
I own two yellowing radioactive K mount Pentax 50/1.4 lenses.

Color of the coating is NOT the sole indicator of thorium in the coatings. Geiger counter can be used in conjunction with several non-thoriated lenses.

It is the glass itself what yellows.

Edit: The radiation numbers and serial numbers are interesting on that chart. It is also interesting that a 20mm lens came up with a very low reading. He drew an conclusion int there somewhere that the 55/1.8 and 55/2 may have different optical formalae due to the radiation levels and that part is a stretch since the biggest difference is the aperture diaphragm.

That would be me, and yes, I think that over the years the 55/1.8 (and optical similar 2.0) versions changed formulae, because they changed the optical glass.

[Blue]

How much proof do you need ?
I own two yellowing radioactive K mount Pentax 50/1.4 lenses.

Then you should have clearly stated that they were K lenses.

It is the glass itself what yellows.

That wasn't my point. The point is that coatings can five lenses the appearances of having a certain hue to them and that the hue alone isn't enough to solely determine radioactivity.

That would be me, and yes, I think that over the years the 55/1.8 (and optical similar 2.0) versions changed formulae, because they changed the optical glass.

There is no indication that Asahi changed the formula of the 55/1.8 lenses nor the 55/2. I will cite Gerjan van Oosten as well as the Asahi Optical Historical Club on that because you are implying changes within SMC 55/18 and SMC 55/2 lenses based on radioactivity. Keep in mind that they would not have to change the formulae, i.e. arrangement and thickness to use or not use thoriated glass for some of the elements. This brings up a question since there were at least to basic ways to thoriate a lens: thorium fluoride coated germanium lens and the homogeneous thorium oxide lens. Which way did Asahi Optical do it?

---

Last edited by Blue; 11-04-2013 at 12:05 PM.

[Blue]

The coating is not the source of the yellowing. I had to sun bleach the lens along with my 1.4 and the yellowing is no longer as severe as in the shot posted.

Let me be clear for the nth time, I am saying that some lenses may have a subtle yellow hue due to coatings. Some have blue hues etc.

How Lens Coating Works

Don't get bored before getting to this section.

"Okay, great, I get it. But why the color?"

[glasbak]

Then you should have clearly stated that they were K lenses.

My fault, I gave a link to a page with too much information, sorry.

There is no indication that Asahi changed the formula of the 55/1.8 lenses nor the 55/2. I will cite Gerjan van Oosten as well as the Asahi Optical Historical Club on that because you are implying changes within SMC 55/18 and SMC 55/2 lenses based on radioactivity. Keep in mind that they would not have to change the formulae, i.e. arrangement and thickness to use or not use thoriated glass for some of the elements. This brings up a question since there were at least to basic ways to thoriate a lens: thorium fluoride coated germanium lens and the homogeneous thorium oxide lens. Which way did Asahi Optical do it?

As far as I know, the optical glass used is as much part of the lens optical formulae, as is curvature or spacing.

I did see a change in radiation in 1.8/55mm takumars from the early to the later 'super takumar' versions (model I and model II according to Gerjan),
Then also all later S-M-C, and SMC 1.8/55mm takumars I tested, appeared to be radioactive, but much to my surprise, the K mount versions were not radioactive.

And about the difference between the 1.8 and 2.0 versions, I know.

Germanium lenses are transparant for IR, not very much so for the visible spectrum.
So, I vote for the homogeneous thorium oxide lens.
Also because the yellowing is worse in the thicker part of the lens elements, that would not be the case if it was only the coating.

[Digitalis]

no evidence of any SMC K ever having thorium. It wasn't put in manyl TAK focal lengths.

And you are basing this statement on what exactly? i'm sure there are a few out there.

Germanium lenses are transparant for IR, not very much so for the visible spectrum.

Pure Germanium lenses are useless for visible light applications, I can recall some Lenses that use elements with Germanium oxides in them.

[Blue]

And you are basing this statement on what exactly? Thorium is very common element in the earths crust - naturally occurring thorium compounds are said to be three times more abundant than tin. Whatever it is in there is a Beta emitter, with a pretty long half life at that.

Where did I say anything about how common the element was and what that has to do with Takumars and K-mount lenses? I simply said it wasn't used in that many focal lengths and models, some of the 50/1.4, 55, a few 35 and the 58/1.8 and maybe weakly the 20/4.5.

[Blue]

My fault, I gave a link to a page with too much information, sorry.



As far as I know, the optical glass used is as much part of the lens optical formulae, as is curvature or spacing.

I did see a change in radiation in 1.8/55mm takumars from the early to the later 'super takumar' versions (model I and model II according to Gerjan),
Then also all later S-M-C, and SMC 1.8/55mm takumars I tested, appeared to be radioactive, but much to my surprise, the K mount versions were not radioactive.

And about the difference between the 1.8 and 2.0 versions, I know.

Germanium lenses are transparant for IR, not very much so for the visible spectrum.
So, I vote for the homogeneous thorium oxide lens.
Also because the yellowing is worse in the thicker part of the lens elements, that would not be the case if it was only the coating.

Then the K-mount ones aren't radioactive?

[Digitalis]

t is the glass itself what yellows.

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

Then the K-mount ones aren't radioactive?

I wouldn't be surprised if some of the early lenses made during the transition between the M42 mount and the K bayonet were radioactive.

Where did I say anything about how common the element was and what that has to do with Takumars and K-mount lenses? I simply said it wasn't used in that many focal lengths and models, some of the 50/1.4, 55, a few 35 and the 58/1.8 and maybe weakly the 20/4.5.

Apologies, thanks for the clarification. I was getting the impression that you were suggesting that thorium wasn't the culprit...that some other, nastier element was responsible.


Thoriated lens element taken with a Pentax K5IIs with my non-radioactive K 55mm f/1.8 - 11mm extension tube used. yes, I admit it...I bite my nails.

[Blue]

Incorrect, It is actually the Canada basalm 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.





I wouldn't be surprised if some of the early ones during the transition between the M42 mount and the K bayonet were radioactive.

It could be possible if some of the elements were left over from earlier builds. However, 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.

[Digitalis]

It could be possible if some of the elements were left over from earlier builds.

You can find some of the Leica M39 mount lens elements that were radioactive appearing in the early M bayonet series during the transition period* (the LEITZ 50mm f/2 summicron collapsible, is a good example of this) - having two separate fabrication lines for the same lens is a bit silly....and Germans don't do silly.

*though good luck finding one, they are a bit of a collectors item. Voigtlander had a few M39 wide angle lenses that were radioactive.