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Interesting claim about Tele-Takumar 300/6.3
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PostPosted: Thu Aug 22, 2024 5:52 pm    Post subject: Interesting claim about Tele-Takumar 300/6.3 Reply with quote
Today I came across an interesting statement about the Tele-Takumar 300mm f/6.3 which has piqued my intellectual curiosity:

Source: https://www.pentaxforums.com/forums/8-film-slrs-compact-film-cameras/137774-how-radioactive-super-takumars-5.html#post2893390

Quote:
I can personally vouch a little-known fact: The Tele-tak 6.3/300 has a single thoriated element. It's buried in the middle of the lens, so it's only obvious when you rebuild it. I think there is another as well. I'll make another post after I check my records...


I was wondering if anyone has any insight about this. What optical purpose would a lens designer have in putting a thorium element in the middle of the lens, where in most such lenses it is either a front and/or rear element? Or perhaps this type of construction is not as uncommon as I think it is but being deep inside a lens it is a lot more difficult to detect?

Regards, Christine


PostPosted: Thu Aug 22, 2024 6:48 pm    Post subject: Re: Interesting claim about Tele-Takumar 300/6.3 Reply with quote
connloyalist wrote:
What optical purpose would a lens designer have in putting a thorium element in the middle of the lens


What do you mean by "in the middle of the lens"? The said Tak has a positive triple as a master lens in the front, and a negative doublet acting as a "tele converter / field flattener" at the rear. In the middle there's a lot of ... air:

https://www.pentaxforums.com/lensreviews/Tele-Takumar-300mm-F6.3.html

The thorium glass probably is one of the two negative lenses since the two positive lenses in the front triplet MUST be made from low dispersive glass (it's a 300mm lens after all) and since - according to your information - it's an "inner lens" (thus not the rear positive lens).

S

Last edited by stevemark on Thu Aug 22, 2024 7:05 pm; edited 1 time in total


PostPosted: Thu Aug 22, 2024 6:59 pm    Post subject: Reply with quote
That's the only reference stating that lens has radioactive element, not Gerjan, not Dru. I have several of those lenses. But no meter.

More interesting to me is the source of your statement " in most such lenses it is either a front and/or rear element". Smile That is not the case with other Takumars. All of the other radioactive Takumars have radioactive internal elements. None have radioactive front and/or/rear elements.


PostPosted: Thu Aug 22, 2024 7:03 pm    Post subject: Re: Interesting claim about Tele-Takumar 300/6.3 Reply with quote
stevemark wrote:
What do you mean by "in the middel of the lens"? The said Tak has a positive triple as a master lens in the front, and a negative doublet acting as a "tele converter / field flattener" at the rear. In the middle there's a lot of ... air:

https://www.pentaxforums.com/lensreviews/Tele-Takumar-300mm-F6.3.html

The thorium glass probably is one of the two negative lenses since the two positive lenses in the front triplet MUST be made from low dispersive glass (it's a 300mm lens after all) and since - according to your information - it's an "inner lens" (thus not the rear positive lens).

S


I looked in Gerjan van Oosten's book (which I received yesterday) to see if he listed a diagram of the optics, but I didn't see it under the listing for this lens. Didn't think to look at the Pentax forum for this (which is an obvious place to look for such a thing). So I was quoting the original statement when I said "the middle of the lens".

From your analysis, it must be the second lens from the rear?

Incidentally, i just noticed the info on the Pentax forum's page differs slightly from Gerjan van Oosten's book. The book has it at 5/4, the website as 5/5. Lens-db.com also states it as 5/5.

Regards, C.


PostPosted: Thu Aug 22, 2024 7:16 pm    Post subject: Reply with quote
visualopsins wrote:
All of the other radioactive Takumars have radioactive internal elements. None have radioactive front and/or/rear elements.


My statement was an assumption based on casual observation of discussions on such topics; I must be more careful supporting my statements in the future, humble apologies.

But interesting, I didn't know that. I would be interested in reading more about the positioning of thorium elements in an optical formula if you could point me to some literature.

Regards, C


PostPosted: Thu Aug 22, 2024 7:17 pm    Post subject: Re: Interesting claim about Tele-Takumar 300/6.3 Reply with quote
connloyalist wrote:

From your analysis, it must be the second lens from the rear?


I don't know. Both the two negative lenses in such [5/5] or [5/4] tele constructions (positive triplet master lens in front, negative doublet at the rear) are usually made from high refractive glass (nD 1.7 ... 1.Cool with a medium to high dispersion (v 50 ... 25). Thorium glass fits into that category.

S
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