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11-01-2013, 07:53 AM   #1
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SMC Pentax K 1.4/50 - yellowing!

I recently bougt a SMC Pentax K 1.4/50 in beautyful condition, the lens looks like new! But while examing the lens closely I noticed a clearly visible yellow tint in the glass.
To me it looks exactly like the yellowing of the Takumars which contain thoriated radioactive glass elements.
I have always thougt the SMC Pentax K does not contain thoriated glass and also should not be radioactive.
Has somebody ever tested the radiation of a SMC Pentax K 1.4/50? Is it radioactive or not? Can the yellowing be healed like it can be done on a SMC Takumar?

Thanks for your answer.

Timo

11-01-2013, 08:23 AM - 1 Like   #2
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Since that trick works on coatings containing thorium, I have my doubts but it is worth a try. Are you sure it isn't the natural color of the coating? Is it having an adverse effect on colors?
11-01-2013, 09:10 AM   #3
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Yes I think the images get a slightly but noticeable yellow color cast and I imagine that it "darkens" the lens also.

- I`ve often read here that the coating of the Takumars where radioactive but as far as I know it is not the coating which contains thorium
but the glass it self, which was blendet with thorium oxide to decrease the diffraction index.


Timo
11-01-2013, 09:19 AM   #4
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Here is an article from Camerapedia on the subject of Thorium Oxide in photographic lens glass from the 40's to the 70's. The article specifically mentions the SMC Takumar 50/1.4 (among others) as a lens known to have tested positive for radioactivity. Since the SMC 50/1.4 (K50/1.4) is ostensibly directly derived from the SMC Takumar version (citation - J. Colwell, SPLOSDB, PP. 10) with a bayonet mount attached, it is possible to imagine an early K50/1.4 (as a transition lens) might have used thoriated glass blanks originally intended for use in the SMC Tak.

I have no specific knowledge to the effect, just cite the possibility.


Last edited by monochrome; 11-01-2013 at 09:29 AM.
11-01-2013, 09:25 AM   #5
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QuoteOriginally posted by monochrome Quote
Since the SMC 50/1.4 (K50/1.4) is ostensibly directly derived from the SMC Tak version, with a new mount attached, it is possible to imagine an early K50/1.4 (as a transition lens) might have used thoriated lens blanks originally intended for use in the SMC Tak.
Just what I was about to say. Possible, but no way to prove/disprove. As I understand it the radioactivity in the glass eventually turns the cement in the lens yellow. I guess it could be the coating instead but I have always thought it was the glue or cement between lens elements.

One way to test would be to expose it to UV as is done with the Takumars. If it clears then I guess you have your answer.

Also, if you have the camera on AWB you should not be seeing any color change on the image.......
11-01-2013, 09:34 AM   #6
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QuoteOriginally posted by jatrax Quote
I guess it could be the coating instead but I have always thought it was the glue or cement between lens elements.
The coatings themselves are very thin layers - are far to thin to have such a massive impact on the light transmission or colour rendering of a lens, the adhesive used between the lenses (canada basalm) is the most likely culprit, because there is considerably more of it, and because of its organic nature exposure to Beta radiation emitted from the lens element is more likely to experience unanticipated detrimental effects.
11-01-2013, 09:40 AM   #7
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QuoteOriginally posted by Digitalis Quote
(canada basalm)
That's what I was trying to remember! I thought it was an organic base adhesive, just could not think of what it was. Thanks.
11-01-2013, 10:50 AM   #8
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QuoteOriginally posted by monochrome Quote
Here is an article from Camerapedia on the subject of Thorium Oxide in photographic lens glass from the 40's to the 70's. The article specifically mentions the SMC Takumar 50/1.4 (among others) as a lens known to have tested positive for radioactivity. Since the SMC 50/1.4 (K50/1.4) is ostensibly directly derived from the SMC Takumar version (citation - J. Colwell, SPLOSDB, PP. 10) with a bayonet mount attached, it is possible to imagine an early K50/1.4 (as a transition lens) might have used thoriated glass blanks originally intended for use in the SMC Tak.

I have no specific knowledge to the effect, just cite the possibility.
However, it was one of the earlier versions of the Super Tak 50/1.4 that had thorium and not many other Taks did.

11-01-2013, 02:04 PM   #9
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11-01-2013, 05:59 PM   #10
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QuoteOriginally posted by Blue Quote
However, it was one of the earlier versions of the Super Tak 50/1.4 that had thorium and not many other Taks did.
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.

http://www.flickr.com/photos/s58y/6802092736/


Steve

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

Last edited by stevebrot; 11-01-2013 at 06:07 PM. Reason: Removed sentence for accuracy
11-01-2013, 07:27 PM   #11
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QuoteOriginally posted by stevebrot Quote
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.
I can't swear that my both my S-M-C Tak and SMC Tak 50/1.4 are actually hot, but both were certainly yellowed and both have certainly been cured on the windowsill.
11-01-2013, 07:59 PM   #12
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QuoteOriginally posted by Blue Quote
t was one of the earlier versions of the Super Tak 50/1.4 that had thorium and not many other Taks did.
The takumar 35mm f/2 allegedly has thoriated lens elements in it. Unfortunately I cannot find my copy of this lens,I have a case and lens hood for it, but no lens.
11-01-2013, 10:11 PM   #13
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QuoteOriginally posted by Digitalis Quote
The takumar 35mm f/2 allegedly has thoriated lens elements in it. Unfortunately I cannot find my copy of this lens,I have a case and lens hood for it, but no lens.
The 49mm version certainly does.

11-01-2013, 10:49 PM   #14
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QuoteOriginally posted by stevebrot Quote
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 indicates that this is not correct.
A radiation meter does not care about what people think, believe or say, it just measures radiation.
11-04-2013, 08:04 AM   #15
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QuoteOriginally posted by Digitalis Quote
The takumar 35mm f/2 allegedly has thoriated lens elements in it. Unfortunately I cannot find my copy of this lens,I have a case and lens hood for it, but no lens.
True, but there is no evidence of any SMC K ever having thorium. It wasn't put in manyl TAK focal lengths.

Last edited by Blue; 11-04-2013 at 08:14 AM.
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