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08-22-2009, 07:50 PM   #1
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Yellowing Super-Tak == SMC?

Hi all,

I have a Super-Tak 50mm f/1.4 . The lens is very yellow, which I read was a result of thorium in the coating.

Does that mean this Super-Tak has the same coating as an SMC-Tak 50mm f/1.4?

Thanks,
-slrl0ver

08-22-2009, 08:09 PM   #2
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QuoteOriginally posted by slrl0ver Quote
Hi all,

I have a Super-Tak 50mm f/1.4 . The lens is very yellow, which I read was a result of thorium in the coating.

Does that mean this Super-Tak has the same coating as an SMC-Tak 50mm f/1.4?

Thanks,
-slrl0ver
I'd say no. While both Super Taks annd SMC Taks exhibit this phenomenon that doesn't mean the coatings are the same.

See, it's the rear element that yellows, and the SMC coatings are more than likely only applied to the front elements at least in these early examples.

Regards,
Mike
08-22-2009, 08:13 PM   #3
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I heard that if you sunbath that baby, it will turn itself clear again.
08-23-2009, 02:44 AM   #4
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It's been my understanding that it's the properties of the "rare earth glass" used in the rear element that causes the yellowing effect over time, not the coatings.

08-23-2009, 02:50 AM   #5
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Copied from A post I made in this thread:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/70034-super-ta...m-f-1-8-a.html

"I have the Super Tack versions of the 50mm f/1.4 and a 55mm f/1.8. They were both yellow when I got them, the 1.4 was a bit worse but they both have about the same level of radioactivity as measured with my cheep geiger counter. It appears that it is the rear lens element that is radioactive so if you intend to leave one in the sun for UV treatment then it should have the back end facing up.
The thorium was added to the glass to raise the refractive index and lower the dispersion of the glass so these lenses used the LD glass of their day. The radioactice decay slowly turns the glass yellow but UV treatment anneals the yellowing out of the glass turning it clear again. The type and level of the radioactivity from these lenses is not really dangerout but it's best not to keep one in you pocket for long periods...
I also have the 135mm f/3.5 and the 300mm f/4 and neither of these are radioactive."

To elaborate on the sunbathing, It is the UV in the sunlight which bleaches the yellowing and leaves the lens clear. I left mine on a windowsil for about 6 months over the summer (we don't see the sun much here in wales) and this removed much of the yellowing. However window glass absorbes a lot of the UV so this will probably have slowed the process. A UV exposure box would be ideal but a few sessions at a local tanning salon may be worth a try
08-23-2009, 02:50 AM   #6
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How To Fix Yellowing

The Spotmatic Group Method
Here is the method we use on the Yahoo! Spotmatic group:

How to Cure Yellowing in Takumar 50mm f1.4 lenses.

Remove both lens caps and wrap the uncapped lens, except the rear element,
in aluminum foil and then set it on a windowsill that faces towards the sun.
Prop it up with something (like a rolled-up towel) so that it tilts in order
to get the most sunlight into it. Then go away and leave it.

A mild case takes about a week to ten days on a windowsill in California.
A severe one takes about a month.
The foil serves two functions. It reflects light off the lens body and
thus reduces heat buildup that otherwise would occur due to the black
finish on the lens and this could affect the lubrication inside the lens.
And by not capping the lens before wrapping it, light makes its way from
the back of the lens and reflects back off the foil at the front of the lens,
thus attacking the yellowing again.

This "sunlight cure" method has been successfully employed by a number of
members of the Spotmatic group and many 50mm f1.4 lenses which were
thought to be useless for colour photography have now been returned to
active service with "water clear" glass.
Some others have reported good results in about three weeks using an
ultraviolet light source.

The yellowing problem affects the Model II 50mm f1.4 Super-Takumars and all
other 50mm f1.4 lens through to the K-mount series.
The older Model I 50/1.4 Super-Takumars, the 8-element ones (which can be
distinguished by the protruding element at the rear NOT having a
protective metal rim), do not turn yellow because they do not have
the radioactive element, using an additional regular optical glass element to
get higher refraction instead. The Yellowing problem also affects the later
f2 35mm lenses with 49mm filters.
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08-23-2009, 03:56 PM   #7
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Window glass (for houses and cars) blocks most of UV light. To confirm this, ask anyone who wears "transitions" lenses.

It took 2 days to cure the yellowing of my Takumar 50mm F/1.4, on a South-facing window sill. It was late spring in No. CA. The lens was outside, not inside behind the glass.

I think it would take months behind a window.

I also saw the yellowing on a Takumar 55mm F/1.8.
08-23-2009, 04:32 PM   #8
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I have my super tak 50mm 1.4 outside now....this is day 1 in my attempt to clear it up. I thinking of taking it into the lab and leaving it under one of the UV lights we use....I'll report back with results in a few days....

mark

08-23-2009, 10:27 PM   #9
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QuoteOriginally posted by SOldBear Quote
I also saw the yellowing on a Takumar 55mm F/1.8.
There is absolutely no reason why a 55mm f1.8 would go yellow. They did not have any rare earth glass elements. I have multiple examples of that focal length and all of them are "water clear".
08-24-2009, 02:03 AM   #10
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QuoteOriginally posted by rayallen Quote
There is absolutely no reason why a 55mm f1.8 would go yellow. They did not have any rare earth glass elements. I have multiple examples of that focal length and all of them are "water clear".
so far as several of us here have admitted, 55s do indeed yellow.

I also have a number of examples, here is my list of 55's:

Auto-Takumar 2/55 - clear
Auto-Takumar (early Super variant) 1.8/55 - very slight yellowing
Super-Takumar 1.8/55 - some yellowing
Super-Multi-Coated Takumar 1.8/55 - yellowing (more than others)
SMC Takumar 1.8/55 - yellowing (not as much as the S-M-C but close)

none of these are anywhere near as yellow as my S-M-C 1.4/50 but it is indeed there. I even noticed a yellow cast when using my 55's when looking through the viewfinder of my istD (something I did not see with my K110D or K-m. its also visible when using the 1.4/50)

another member used a geiger counter and confirmed radioactivity with a 55. Asahi Optical may have only ever publicly announced use of thorium in the 1,4/50 but that doesn't mean that was the only lens they used it in.
08-24-2009, 03:34 PM   #11
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QuoteOriginally posted by séamuis Quote
so far as several of us here have admitted, 55s do indeed yellow.

Another member used a geiger counter and confirmed radioactivity with a 55. Asahi Optical may have only ever publicly announced use of thorium in the 1,4/50 but that doesn't mean that was the only lens they used it in.
Well, that is the first I have heard of that with 55s. Thanks for letting me know.

I will certainly keep checking the ones that I have.
08-24-2009, 07:32 PM   #12
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Of Pentax's lenses, the 55mms are the most radio active but do not suffer the same fate as the 50mm f1.4s from the second version through the SMC Pentax 50mm f1.4 which are known to yellow.

Any perceived yellowing of the 55s are more a feature of the coating at the time as they were changing going from the original single coat to multi-coating. For a quick check for yellowing, compare the colour of the light passing though the lens on a white piece of paper. You will find that there may some colouration and that is a design feature. German lenses of the time were noted as being cooler than the Japanese lenses.

I have some 18 or so 55mms from the very early 2.2 and 2.0 Auto-Takumars through the SMC Pentax 55mm f1.8 K mount and I have not seen any colouration that is typical of the 50mms. In fact, the SMC Takumar and the SMC Pentax 55s are among my all time favourite lenses!
08-25-2009, 05:17 AM   #13
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QuoteOriginally posted by bobrapp Quote
Of Pentax's lenses, the 55mms are the most radio active but do not suffer the same fate as the 50mm f1.4s from the second version through the SMC Pentax 50mm f1.4 which are known to yellow.

Any perceived yellowing of the 55s are more a feature of the coating at the time as they were changing going from the original single coat to multi-coating. For a quick check for yellowing, compare the colour of the light passing though the lens on a white piece of paper. You will find that there may some colouration and that is a design feature. German lenses of the time were noted as being cooler than the Japanese lenses.

I have some 18 or so 55mms from the very early 2.2 and 2.0 Auto-Takumars through the SMC Pentax 55mm f1.8 K mount and I have not seen any colouration that is typical of the 50mms. In fact, the SMC Takumar and the SMC Pentax 55s are among my all time favourite lenses!
I have Takumar 55's that old as well, and I have not seen multiple examples of an 'amber' single coating. only the very earliest had single coating. even though they were not "SMC" if im not mistaken, just about all Super-Takumars were indeed multi-coated. as Asahi Pentax made a very clear distinction between multi-coating and SMC. lack of SMC does not equal lack of multi-coating. and we just don't know where the transitions were or what strange things Asahi Optical tried. for example, most people do not know that the FA 43mm Limited has a special 'ghostless' coating, a coating in which Asahi Optical first started developing in the 60's. Im not going to claim that I have 55's with thorium, but I do have 55's that exhibit yellowing. and since none of us really know the reason why or what thing Asahi Optical tried with their lenses its impossible to say one way or the other on any assumption.
08-31-2009, 11:29 AM   #14
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clear as new...

QuoteOriginally posted by forensicscientist Quote
I have my super tak 50mm 1.4 outside now....this is day 1 in my attempt to clear it up. I thinking of taking it into the lab and leaving it under one of the UV lights we use....I'll report back with results in a few days....

mark

left my super tak 1.4 under a UV light here in the lab (100watt longwave UV). Happy to report, that in just over 48 hours, its perfectly clear..... Wow, what a difference......
10-02-2009, 02:19 PM - 1 Like   #15
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Wow, so glad to learn of this UV trick. Now I want some lenses.

Oh, btw, can we please tell everyone that the radiation is bad for them and they should avoid Takumars at all costs?


The prices for them are too high.

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