[stevebrot]

It's definitely very colored when looking at it. Looking through it it's hard to tell and I can't mount it because of the stupid metal tab on the thing that I need to file down.

The coatings on the Fujinon 55/1.8 reflect back a golden yellow in the photos I found.* If it does not appear smokey yellow looking through, it is probably fine. The photo below shows the view through a yellowed radioactive S-M-C Takumar 50/1.4 to a white background beyond:


Photo lifted from mflenses.com: LINK


Steve

* Despite some reports on the Web, the radioactive thorium is in the glass itself and is not part of the coating.

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Last edited by stevebrot; 03-23-2017 at 04:25 PM.

[boriscleto]

It may be noted that some lanthanum-doped lenses (e.g. Industar 61 L variants) are measurably radioactive, though probably not from the weakly radioactive lanthanum. It is thought that thorium was also added to the mix as part of the "secret sauce".

With the Soviet lenses I've always wondered if they might have been irradiated after the Chernobyl disaster...

[cyberjunkie]

IIRC, the yellowing in lanthanum glasses is due to thorium impurities. Soviet Union lenses with lanthanum optical glass usually don't show any yellowing (I've never seen one), while many Japanese optics do. The reason is simple: the lanthanum oxide came from different mines. Optical glass engineers in the Soviet Union had plenty of raw materials, including rare earth elements, coming from local mines, while the Japanese had to import from abroad. If I'm not mistaken, lanthanum came from South America (Chile?), and contained thorium impurities.
I am not aware of any 35mm lens with a relevant level of radioactivity, or with thorium elements. Thorium glass elements (generally one, sometimes two) were common in high speed aerial lenses used during WWII (and I guess also Korea war). For the reasons already explained, and for the technological advancement in the optical industry post WWII, thorium elements were abandoned in favour of other solutions. BTW, thorium glasses showed a quick decay (yellowing): way less than ten years.
Of course a heavily yellowed lens is actually slower (higher T number), another good reason to move on to other optical layouts with different glasses.

This is what I know... or at least what I remember
I collected large format lenses for quite a long time, and I own a few aerial lenses made during WWII or immediately afterwards. This is why I got curious about the use of thorium glasses, and I did a bit of research about that.
Not so sure about my memory, though If I wrote anything incorrect, please correct me.
I'm still curious about this matter

Cheers

Paolo