Pentax Takumar Lens Radiation for Pentax 6x7 Lenses

Discussion in 'Medium Format' started by daniel_katz|2, Sep 22, 2015.

  1. Hello All!

    I've been doing photography for 15+ years now. I shoot 35mm, Medium Format and Large Format. For most of my fashion and portrait work I use a Hasselblad 501C currently but am looking to get a Pentax 6x7, 67 or 67II with a 105mm F2.5 but I read on this site that the Takumer 105mm f2.5 is made with radioactive Thorium glass.

    My question is whether this is for ALL the 105mm f2.5 lenses or only the older Takumar branded ones (the newer ones that just say Pentax are not radioactive)? When did Pentax switch over to non-thorium lens elements? How can I know which ones don't have it? Were there any other lenses that were radioactive?

    Also I am debating between buying the three different Pentax camera's. The II is obviously more expensive but I want a really bright WHITE viewfinder. I can't stand it when they have the split finder and the yellowing. Do all of them have that? Thank you!
    The reason for switching is that I really love the Bokeh, Contrast and look you get with teh 105mm F2.5 Takumar, it's very similar to the 110MM/F2 Hasselblad Zeiss lens. I could buy a dozen Pentax 67 camera and lens combo's for the price of the 110 zeiss hasselblad combo!​
  2. SCL


    Are you just curious about the radiation or are you concerned about it from a health standpoint. The best way to determine the level of radioactivity, of course, is to measure it with a geiger counter. But everything on the earth has inherent radioactivity, it is an issue of relativity. The level from the lenses is no more than what you receive from wearing a watch with luminous dials/hands,,,basically nothing to worry couldn't live long enough for it to become an issue. However...the radioactivity in the lenses, over time, has affected the color/transparency of some lenses...usually slightly yellowing or browning them. Depending on your point of view, this is great or something to be remedied, which is quite merely expose the lenses (or viewfinders in your case) to UV or sunlight for a week or so. I've done it with 2 lenses and they are crystal clear...which is what I wanted. Some photographers, however prefer the color tint to their shots or the increased contrast for B&W film instead of an external filter. Sorry I can't answer your question about ALL 105/2.6 lenses or exactly when Pentax switched over. In the 1950s-1970s it wasn't uncommon for leading mfrs of fast lenses to incorporate radioactive elements into the glass formulas to achieve what they hadn't been able to do with standard glass formulas, as certain glasses became less available after the wars.
  3. There have been a lot of lenses using Thorium Oxide and/or Lanthanum glass (and some eye pieces too, a situation more likely to be hazardous).

    Here is a link to a short article that includes an extensive list of such lenses:
  4. I'd question the accuracy of that Camerapedia article WRT the type of emissions from Thorium-bearing glass. What I've stated below has been verified by examination of a known Thorium-glass lens using a calibrated radiation detector and in collaboration with an expert radiation-protection consultant.
    If the lens is still water-clear, then there's a good chance it isn't radioactive to any hazardous degree.
    Thorium - an Alpha emitter - decays over time into daughter products that are Gamma emitters (not Beta). It's when this happens (over decades) that the lens becomes hazardous. Alpha particles can be blocked by a thin metal lens cap. Gamma particles go on and on like the proverbial battery bunny, and will even pass through a Lead brick. Those radioactive particle emissions also change the structure of the glass and it darkens to a light amber colour over time.
    Alpha particles can cause more tissue damage than Gamma particles, but only when in close proximity. It's the penetrating ability of Gamma radiation that poses a greater hazard.
    In short; if there's no browning of the glass then no worries! Although it's reported that the discolouration can be reversed by exposure to strong UV. I'm doubtful of that myself, but it may be so.
  5. paul ron

    paul ron NYC

    wow thanks for the info. a real eye opener since
    so many of us baby boomers own spotmatics n
    yellow lenses.

    btw the yellowing of the glass actually boosted
    the contrast n rendered reds better in old film
    emulsions back then. elliminated the need to
    use a yellow filter which was customary then.
  6. FWIW, a few years ago I posted a side-by-side test of a 1970s Canon thorium-containing lens versus two newer lenses. This lens was moderately discolored. You can see the results here. (Those are uncorrected scans.)
  7. Someone from the other forums said that it was only the Super Takumar. That the newer lenses shouldn't have any Thorium... I guess a good way to know is to buy one and buy a geiger counter to test it!
  8. That was me that said that Thorium was only in the Spuper-Takumar 105. Trust me on this one. The SMC Takumar and Pentax 105s did not use it.
  9. Thank you Steve! That's all I wanted to know! None of the other Medium Format Pentax lenses should have it either then?
  10. Between the 6x7 and 645 lenses, the only lens I know of with Thorium is the 105mm Super-Takumar.
  11. Thank you Steve - you've been a huge help! Can't wait to get a Pentax 67 with a SMC 105 2.4 lens now! :)
  12. Anyway thank you for all your answers - you've been a huge help! If anyone has a Pentax 6x7, 67, or 67II with a SMC Pentax 105 2.4 - please PM me as I'm looking for one and would be open to trading for my drum scanning services.
  13. Hey all.
    I can attest to the fact that exposure to UV HAS cleared up a yellowed lens for me. I had earlier contacted Gus Lazzari regarding my yellowed Canon FD Breechlock 35mm f2 lens, S/N 528xx. He told me that not only did he have the same lens, but to take mine and place it in a window or dashboard of car for a few weeks and that the UV light should help the problem. It was the darndest thing to hear that, but I placed the lens on a shelf window so it would get all-day sun and left it there for several weeks (five I believe?) I did turn it two to three times a day to follow the sun. (I placed the rear of the lens facing the sun so the lens would not focus the sun to a pinpoint and start a possible fire.) To my astonishment, after several weeks of treatment, the glass became clear again, and as near as I could tell was very close to matching all my other lenses.
    So YES, I have verified for myself that UV exposure CAN clear a yellowed lens. Now keep in mind that various other lenses, due to construction and number of elements, may have varying results from this process. All I DO know for certain is this worked on my Canon FD 35mm. Apparently the earlier FD 35mm f2 lenses, with the sort of flat-ish or concave front element, are the culprits for having radioactive glass.
    I do have a bit of concern about long term use of any radioactive lenses, but I seldom used my 35mm f2.
  14. But wouldn't the UV light be filtered out by the glass in the windows? Maybe it is just photons that help rather than UV photons...

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