Radioactive Pentax Super Takumar 50mm f/1.4

To those who follow my blog (thank you), you will know I am a huge fan of vintage glass. There was a time I had more manual lenses than I did native mount, autofocus gear. Call me glutton for torture or a hipster, I just find them extremely fun to use. After spending time in different photography forums, I would hear people talk about the one glass every vintage prime lens fan should own and that is the Super Takumar 50mm f/1.4 (the m42 mount variant). When I took delivery of this lens, I was shocked to find a very noticeable yellow tint. I originally thought it was a defective unit but after a little bit of googling I later discovered the yellowing is a byproduct of the radioactive gas within the lens itself. That's right, RADIOACTIVE. According to www.pentax-manuals.com this is what is causing it: 

Apparently the atomic structure of the glass is altered by radiation from trace amounts of radioactive thorium in one rare-earth glass element, creating the yellowish-brown color; and this change is reversed by exposure to ultraviolet light.

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. 

Untreated glass straight from the seller

Lucky for me I work at a facility where I have access to a portable UV light device. With the help of my fellow co-worker, we decided to conduct our very own un-scientific experiment with an extremely poor documentation process. Because this was done at work, I did not have my camera handy. Instead we decided to use his iPhone to record the progress after each day. While I understand we did not use the same white balance and camera settings for each shot, I can attest for the differences and the pictures are demonstrative of our findings. The UV light device had two settings for long and short wave. Without any kind of research we started with a 24-hour treatment of short-wave ultra violet light. It seemed to have lighten the yellow tint but it's still clearly present.

After 24 hours of direct long-wave UV light treatment

Short wavelength UV light seems to be good at killing bacteria but ineffective at passing through glass, plastic and the earth's atmosphere. The sun is a great source of UV light and is one of the known ways to treat thorium tinted glass, but requires a more intricate setup and exposes your lens to the elements - something that can do more harm than good to an old lens like the Super Takumar.  We decided to change the setting to long wave UV light and it had a greater impact.

After 24 hours of long wavelength ultra violet light
Total 48 hours of exposure

After another 24 hours of long wavelength ultra violet light
Total 72 hours of exposure

As you can see, the Super Takumar lens has been cured of its yellow tinting. Part of me wishes I could have taken sample photos of a test chart showing the effect of the thorium tint, but this was done on a whim. I thought this was a neat little experiment that hopefully can help others wanting to rid themselves of this “problem”. I hope this helps and happy shooting.

This was the model UV Light used for the "experiment"

Note: I have seen others use an Ikea lamp with great success. I can't say for certain the UV light is what removed the tinting but it definitely yielded favorable results after our 72 hour treatment.