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Pentax Yellow Lenses
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PostPosted: Wed May 20, 2020 9:24 am    Post subject: Pentax Yellow Lenses Reply with quote

Hi there!

I'm trying to find information about Pentax-K lenses with elements that get yellow because of Thorium.
So far, the only lens everybody confirmed that it gets yellow is the K 50/1.4.
About the K 35/2, instead, I've received mixed information. I've tested 4 copies (SN 52XXXXX) and they were all clean, but many users online report otherwise.

Anybody in the knowledge of Pentax manual lenses?


PostPosted: Wed May 20, 2020 9:37 am    Post subject: Reply with quote

I don't think so. That lens was specifically made for the K-mount. Not an older rehoused Takumar design. By then Pentax didn't use thorium anymore. The 50/1.4 is a Takumar design.


PostPosted: Wed May 20, 2020 10:33 am    Post subject: Reply with quote

D1N0 wrote:
I don't think so. That lens was specifically made for the K-mount. Not an older rehoused Takumar design. By then Pentax didn't use thorium anymore. The 50/1.4 is a Takumar design.


Yes, I don't think there are many Pentax lenses that use thorium.
Maybe the Pentax K 50mm f1.4 is the only possibility, and then maybe not.
From the Takumars and Super Takumars there are many more.
Interestingly, there are so many lenses that used thorium.... Canon FL and FD's, Zeiss, Kodak, Fuji, Olympus, Yashica, Voigtlander.... mostly pre 1980's
Others used lanthanum .
I have just realised that I have quite a few in my own collection that would make a geiger counter tick.
Smile
Tom


PostPosted: Wed May 20, 2020 2:47 pm    Post subject: Reply with quote

The SMC Pentax 85/1.8 should be also radioactive since it's virtually the same lens as the S-M-C Takumar 85/1.8. However, I have no proof. It would be nice if somebody could test this one.

Also the SMC Pentax 50/1.2 come to mind as a very fast K-era design. Did it use thorium or not?


PostPosted: Wed May 20, 2020 6:15 pm    Post subject: Reply with quote

y wrote:
The SMC Pentax 85/1.8 should be also radioactive since it's virtually the same lens as the S-M-C Takumar 85/1.8. However, I have no proof. It would be nice if somebody could test this one.

Also the SMC Pentax 50/1.2 come to mind as a very fast K-era design. Did it use thorium or not?


First I've heard of S-M-C Takumar 85/1.8 being radioactive or yellowing. Mine is clear. So was my S-M-C 85/1.9 and my Super-Takumar 85/1.9.. The only Takumars I know from experience to yellow are Super- and S-M-C 35/2, 7/6 Super- and S-M-C 50/1.4, and Super- and S-M-C 55/1.8.


PostPosted: Wed May 20, 2020 7:49 pm    Post subject: Reply with quote

visualopsins wrote:
First I've heard of S-M-C Takumar 85/1.8 being radioactive or yellowing. Mine is clear. So was my S-M-C 85/1.9 and my Super-Takumar 85/1.9.. The only Takumars I know from experience to yellow are Super- and S-M-C 35/2, 7/6 Super- and S-M-C 50/1.4, and Super- and S-M-C 55/1.8.

The S-M-C Takumar 85/1.8 is indeed thoriated, the Sonnar-ish Takumar 85/1.9 is not (I've measured my Super Takumar 85/1.9). I got no idea about the yellowing phenomenon.

Our member no-x measured many M42 lenses ten years ago. Apart from the 85/1.8 he also discovered the S-M-C Macro 50/4 as being very mildly radioactive. I can confirm this by my own measurement.
http://forum.mflenses.com/radioactivity-of-old-manual-lenses-t25714.html


PostPosted: Wed May 20, 2020 8:34 pm    Post subject: Reply with quote

y wrote:
visualopsins wrote:
First I've heard of S-M-C Takumar 85/1.8 being radioactive or yellowing. Mine is clear. So was my S-M-C 85/1.9 and my Super-Takumar 85/1.9.. The only Takumars I know from experience to yellow are Super- and S-M-C 35/2, 7/6 Super- and S-M-C 50/1.4, and Super- and S-M-C 55/1.8.

The S-M-C Takumar 85/1.8 is indeed thoriated, the Sonnar-ish Takumar 85/1.9 is not (I've measured my Super Takumar 85/1.9). I got no idea about the yellowing phenomenon.

Our member no-x measured many M42 lenses ten years ago. Apart from the 85/1.8 he also discovered the S-M-C Macro 50/4 as being very mildly radioactive. I can confirm this by my own measurement.
http://forum.mflenses.com/radioactivity-of-old-manual-lenses-t25714.html


Thank you!


PostPosted: Wed May 20, 2020 11:53 pm    Post subject: Reply with quote

Thank you very much for your answers.

The Takumar 85/1.8 is in fact radioactive, but I've never seen or heard of a yellowed copy, so I suppose that the Pentax 85/1.8 doesn't get yellow either. The two copies I had of that lens were both clear.
The K 50/1.2 is not radioactive. It's actually fascinating how they managed to create such a fast and high performing lens without using extreme glasses.

So far, the only Pentax lens that I found yellowed is the K 50/1.4, but I need to verify a few other copies of the K 35/2, maybe with an earlier serial number, since that lens is rumored to employ thoriated glass...


PostPosted: Thu May 21, 2020 5:29 pm    Post subject: Reply with quote

Lucio wrote:

The K 50/1.2 is not radioactive. It's actually fascinating how they managed to create such a fast and high performing lens without using extreme glasses.


Of course they did use "extreme" glass (which means high refraction and relatively low dispersion). It just was glass without Thorium!

The "noctilux glass" or the glass used for the Canon 1.2/50mm L both have a much higher refraction than the Thorium glass previously used in fast lenses. A typical thorium glass (used in the early Leitz Summicrons 2/50mm) had a nD=1.691 and v=54.8. This glass later was replaced by the thorium-free LaK9 with nD=1.694 and v=54.6.

Stephan

EDIT I don't know the glass used in the Pentax 1.2/50mm, but the Minolta 1.2/50mm has three lenses with nD 1.79 and v=45.7, one lens with nD=1.79 and v=42.8, and one lens with nD=1.75 and v=50.1; the Canon nFD 1.2/50mm L (which was more than twice as expensive as the Minolta) has one lens with nD 1.88 / v=40,8, two lenses with nD=1.81 and 1.80 and v=46.6, and one lens with 1.77/49.6.


PostPosted: Thu May 21, 2020 6:05 pm    Post subject: Reply with quote

stevemark wrote:
I don't know the glass used in the Pentax 1.2/50mm, but the Minolta 1.2/50mm has three lenses with nD 1.79 and v=45.7, one lens with nD=1.79 and v=42.8, and one lens with nD=1.75 and v=50.1; the Canon nFD 1.2/50mm L (which was more than twice as expensive as the Minolta) has one lens with nD 1.88 / v=40,8, two lenses with nD=1.81 and 1.80 and v=46.6, and one lens with 1.77/49.6.

Do you have a sense how the costs mount in relation to refractive and dispersion values?
We often hear about how costly were some lenses to produce, but I've never seen any sort of breakdown in relation to glass properties.


PostPosted: Thu May 21, 2020 8:31 pm    Post subject: Reply with quote

aidaho wrote:
stevemark wrote:
I don't know the glass used in the Pentax 1.2/50mm, but the Minolta 1.2/50mm has three lenses with nD 1.79 and v=45.7, one lens with nD=1.79 and v=42.8, and one lens with nD=1.75 and v=50.1; the Canon nFD 1.2/50mm L (which was more than twice as expensive as the Minolta) has one lens with nD 1.88 / v=40,8, two lenses with nD=1.81 and 1.80 and v=46.6, and one lens with 1.77/49.6.

Do you have a sense how the costs mount in relation to refractive and dispersion values?
We often hear about how costly were some lenses to produce, but I've never seen any sort of breakdown in relation to glass properties.


No, i don't.

However I have some figures from book production. Printing a book which finally is sold for EUR 50.-- may well cost considerably less than 5.-- EUR. Since the effective production costs are multiplied several times with certain factors, a small change in production (=printing) costs will have a huge effect on the final selling price of a book. Therefore there's an incredible pressure to print as cheap possible, even though often a small increase in printing cost (eg from EUR 5.-- to EUR 5.50) would substantially increase the quality of the product, for instance by using special colors. However that would mean that the selling price is rising from EUR 50 to EUR 55.-- (and not to EUR 50.50 as one reasonably would assume!!). BTW that's one of the reasons which forced me to produce my own books; I did not want to sacrifice quality because the publisher was constantly interfering with my own quality standards ...

I suspect that similar mechanisms are determining the final selling price of a lens; a very small reduction in production costs probably results in a significantly lower selling price ...

S


PostPosted: Thu May 21, 2020 9:48 pm    Post subject: Reply with quote

Obviously, when I say that for the optical scheme of the Pentax K 50/1.2 no "extreme" glass was used I don't mean they didn't use high refraction/low dispersion glass, that would be crazy, I just mean they used the same kinds of glass they put in slower lenses (Dense Flint, Lanthanaum Crown and friends). Something similar happened for the Zuiko 55/1.2, but it's rare. There was also a prototipe of the Pentax K 50/1.2 that implemented (at least on paper) a special variable diffraction element, with "monotonous refractive index variation in the direction of an optical axis of the lens". That's indeed something I would have loved to see... Smile


PostPosted: Thu May 21, 2020 9:58 pm    Post subject: Reply with quote

stevemark wrote:
aidaho wrote:
stevemark wrote:
I don't know the glass used in the Pentax 1.2/50mm, but the Minolta 1.2/50mm has three lenses with nD 1.79 and v=45.7, one lens with nD=1.79 and v=42.8, and one lens with nD=1.75 and v=50.1; the Canon nFD 1.2/50mm L (which was more than twice as expensive as the Minolta) has one lens with nD 1.88 / v=40,8, two lenses with nD=1.81 and 1.80 and v=46.6, and one lens with 1.77/49.6.

Do you have a sense how the costs mount in relation to refractive and dispersion values?
We often hear about how costly were some lenses to produce, but I've never seen any sort of breakdown in relation to glass properties.


No, i don't.

However I have some figures from book production. Printing a book which finally is sold for EUR 50.-- may well cost considerably less than 5.-- EUR. Since the effective production costs are multiplied several times with certain factors, a small change in production (=printing) costs will have a huge effect on the final selling price of a book. Therefore there's an incredible pressure to print as cheap possible, even though often a small increase in printing cost (eg from EUR 5.-- to EUR 5.50) would substantially increase the quality of the product, for instance by using special colors. However that would mean that the selling price is rising from EUR 50 to EUR 55.-- (and not to EUR 50.50 as one reasonably would assume!!). BTW that's one of the reasons which forced me to produce my own books; I did not want to sacrifice quality because the publisher was constantly interfering with my own quality standards ...

I suspect that similar mechanisms are determining the final selling price of a lens; a very small reduction in production costs probably results in a significantly lower selling price ...

S


That makes sense, product sells for 10x production cost, increase production cost by 10%, to increase price by similar 10%.

The Book Industry comparison falls apart very quickly given initial preprint sales of new books, reprints which cost less to print because most of the work is already done, and "just-in-time" printing of single copies a la Amazon, etc. -- have you discussed a paperback edition with your pub? Smile


PostPosted: Fri May 22, 2020 6:56 am    Post subject: Reply with quote

y wrote:
The SMC Pentax 85/1.8 should be also radioactive since it's virtually the same lens as the S-M-C Takumar 85/1.8. However, I have no proof. It would be nice if somebody could test this one.

Also the SMC Pentax 50/1.2 come to mind as a very fast K-era design. Did it use thorium or not?


Yes, the 85mm f1.8 is listed as radioactive, but not the 1.2.

https://camerapedia.fandom.com/wiki/Radioactive_lenses


PostPosted: Fri May 22, 2020 8:51 am    Post subject: Reply with quote

kymarto wrote:
Yes, the 85mm f1.8 is listed as radioactive, but not the 1.2.

https://camerapedia.fandom.com/wiki/Radioactive_lenses

Sorry, I can't see the K-series of SMC Pentax 85mm f1.8 listed there.


PostPosted: Fri May 22, 2020 1:38 pm    Post subject: Reply with quote

I cleared the Yellowing of my two 50/1.4 Pentax Super-Taks using a UV light (NOT a UV-C). I picked up a Hot-Glass Summicron that had been sun-cured, found the lubricants where baked in due to the heat. The UV light puts out much less heat. I set the lens and lamp in a box, let run for a few days. On the Pentax lenses: the rear module is easily removed from the lens, and is where the yellowing occurs. I unscrew them, then place in the box with the lamp.

They are fine lenses.


PostPosted: Fri May 22, 2020 3:38 pm    Post subject: Reply with quote

My K 50/1.4 has zero yellowing, I don't think it's been doped with Thorium.


PostPosted: Fri May 22, 2020 4:01 pm    Post subject: Reply with quote

Lightshow wrote:
My K 50/1.4 has zero yellowing, I don't think it's been doped with Thorium.


Yet my clear m42 50/1.4 and 55/1.8 have zero yellowing, are known to have thorium. One yellowed cleared completely with treatment.


PostPosted: Fri May 22, 2020 4:22 pm    Post subject: Reply with quote

Lightshow wrote:
My K 50/1.4 has zero yellowing, I don't think it's been doped with Thorium.


Mmh... that's weird. All of the copies I tested and heard of are yellowed... may I ask the serial number of your copy?


PostPosted: Fri May 22, 2020 5:25 pm    Post subject: Reply with quote

stevemark wrote:

-clip-

EDIT I don't know the glass used in the Pentax 1.2/50mm, but the Minolta 1.2/50mm has three lenses with nD 1.79 and v=45.7, one lens with nD=1.79 and v=42.8, and one lens with nD=1.75 and v=50.1; the Canon nFD 1.2/50mm L (which was more than twice as expensive as the Minolta) has one lens with nD 1.88 / v=40,8, two lenses with nD=1.81 and 1.80 and v=46.6, and one lens with 1.77/49.6.


Are these figures from a patent or have they been determined on an optical bench through analysis? How exactly does that work? And can you say that one lens is better than the other? The glass used in each the Minolta and Canon seems to be designed towards similar targets/levels of performance.

Lucio wrote:
There was also a prototipe of the Pentax K 50/1.2 that implemented (at least on paper) a special variable diffraction element, with "monotonous refractive index variation in the direction of an optical axis of the lens". That's indeed something I would have loved to see... Smile


That sounds really cool and inventive, but I guess it is extremely difficult to manufacture reliably. Nikon (I believe) had challenges for a long time developing more advanced coatings where, because the refractive index of each layer has to vary to other layers by refractive index, and then also vary according to the properties of the glass, ions were actually diffusing through the layers of different materials, and ruining their effectiveness.


PostPosted: Fri May 22, 2020 7:35 pm    Post subject: Reply with quote

fiftyonepointsix wrote:
I cleared the Yellowing of my two 50/1.4 Pentax Super-Taks using a UV light (NOT a UV-C). I picked up a Hot-Glass Summicron that had been sun-cured, found the lubricants where baked in due to the heat. The UV light puts out much less heat. I set the lens and lamp in a box, let run for a few days. On the Pentax lenses: the rear module is easily removed from the lens, and is where the yellowing occurs. I unscrew them, then place in the box with the lamp.

They are fine lenses.


I guess it bears repeating any time this comes up: You do not need UV-anything to clear a yellowed lens. Any visible light will work.


PostPosted: Sat May 23, 2020 12:05 am    Post subject: Reply with quote

y wrote:
kymarto wrote:
Yes, the 85mm f1.8 is listed as radioactive, but not the 1.2.

https://camerapedia.fandom.com/wiki/Radioactive_lenses

Sorry, I can't see the K-series of SMC Pentax 85mm f1.8 listed there.


I thought that most SMC lenses, as opposed to Super Takumar, were K. In any case the K's were marked SMC, so what is listed might or might not be K mounts.


PostPosted: Sat May 23, 2020 12:10 am    Post subject: Reply with quote

Teemō wrote:
stevemark wrote:

-clip-

EDIT I don't know the glass used in the Pentax 1.2/50mm, but the Minolta 1.2/50mm has three lenses with nD 1.79 and v=45.7, one lens with nD=1.79 and v=42.8, and one lens with nD=1.75 and v=50.1; the Canon nFD 1.2/50mm L (which was more than twice as expensive as the Minolta) has one lens with nD 1.88 / v=40,8, two lenses with nD=1.81 and 1.80 and v=46.6, and one lens with 1.77/49.6.


Are these figures from a patent or have they been determined on an optical bench through analysis? How exactly does that work? And can you say that one lens is better than the other? The glass used in each the Minolta and Canon seems to be designed towards similar targets/levels of performance.

Lucio wrote:
There was also a prototipe of the Pentax K 50/1.2 that implemented (at least on paper) a special variable diffraction element, with "monotonous refractive index variation in the direction of an optical axis of the lens". That's indeed something I would have loved to see... Smile


That sounds really cool and inventive, but I guess it is extremely difficult to manufacture reliably. Nikon (I believe) had challenges for a long time developing more advanced coatings where, because the refractive index of each layer has to vary to other layers by refractive index, and then also vary according to the properties of the glass, ions were actually diffusing through the layers of different materials, and ruining their effectiveness.


It is absolutely not necessary to use thorium doped glass. It is also possible to use lenses with higher curvatures or other glasses with high indices of refraction, though they are usually more costly. Thorium was a way to make fast lenses with low CA more economically, as the crystalline structure of thorium oxide resembles that of fluorite, but is way less expensive and rare.


PostPosted: Sat May 23, 2020 8:47 am    Post subject: Reply with quote

Mike Deep wrote:
fiftyonepointsix wrote:
I cleared the Yellowing of my two 50/1.4 Pentax Super-Taks using a UV light (NOT a UV-C). I picked up a Hot-Glass Summicron that had been sun-cured, found the lubricants where baked in due to the heat. The UV light puts out much less heat. I set the lens and lamp in a box, let run for a few days. On the Pentax lenses: the rear module is easily removed from the lens, and is where the yellowing occurs. I unscrew them, then place in the box with the lamp.

They are fine lenses.


I guess it bears repeating any time this comes up: You do not need UV-anything to clear a yellowed lens. Any visible light will work.


You're correct and it can also be cured by annealing the glass again (not recommended lol), but not all wavelengths interact with glass in the same way nor do all wavelengths have the same energy. That optical glass is generally opaque to UV might have something to do with it, because that light is not being reflected or transmitted, it's being absorbed. UV is also a high-frequency wavelength, so it carries more energy and high-intensity UV lamps are readily available.
The radiation from the thorium adds electrons to the odd valence of the other glass atoms, which then absorbs light in the visible spectrum. So you could use visible light but would probably need a much greater wattage than you'd need with a UV-C lamp.

kymarto wrote:


It is absolutely not necessary to use thorium doped glass. It is also possible to use lenses with higher curvatures or other glasses with high indices of refraction, though they are usually more costly. Thorium was a way to make fast lenses with low CA more economically, as the crystalline structure of thorium oxide resembles that of fluorite, but is way less expensive and rare.


Indeed, there are many methods, each with their own problems. There is no magic bullet, unfortunately. More specifically, it's thorium dioxide.


PostPosted: Sat May 23, 2020 9:07 am    Post subject: Reply with quote

Teemō wrote:
Mike Deep wrote:
fiftyonepointsix wrote:
I cleared the Yellowing of my two 50/1.4 Pentax Super-Taks using a UV light (NOT a UV-C). I picked up a Hot-Glass Summicron that had been sun-cured, found the lubricants where baked in due to the heat. The UV light puts out much less heat. I set the lens and lamp in a box, let run for a few days. On the Pentax lenses: the rear module is easily removed from the lens, and is where the yellowing occurs. I unscrew them, then place in the box with the lamp.

They are fine lenses.


I guess it bears repeating any time this comes up: You do not need UV-anything to clear a yellowed lens. Any visible light will work.


You're correct and it can also be cured by annealing the glass again (not recommended lol), but not all wavelengths interact with glass in the same way nor do all wavelengths have the same energy. That optical glass is generally opaque to UV might have something to do with it, because that light is not being reflected or transmitted, it's being absorbed. UV is also a high-frequency wavelength, so it carries more energy and high-intensity UV lamps are readily available.
The radiation from the thorium adds electrons to the odd valence of the other glass atoms, which then absorbs light in the visible spectrum. So you could use visible light but would probably need a much greater wattage than you'd need with a UV-C lamp.

kymarto wrote:


It is absolutely not necessary to use thorium doped glass. It is also possible to use lenses with higher curvatures or other glasses with high indices of refraction, though they are usually more costly. Thorium was a way to make fast lenses with low CA more economically, as the crystalline structure of thorium oxide resembles that of fluorite, but is way less expensive and rare.


Indeed, there are many methods, each with their own problems. There is no magic bullet, unfortunately. More specifically, it's thorium dioxide.
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