Radioactive Pentax Lens

Don’t Sleep with Yellow Lenses, Darlin’

Photographers who remember the Petula Clark 1967 hit single that’s paraphrased in the title will also probably know what it’s talking about! Regardless of how much one might love the above highly respected lens (and others of its era), try to resist sleeping with them under one’s pillow! You may also want to avoid hanging them on cameras around your neck for extended periods.

They’re radioactive. And I thought it would be good to share this story with the growing band of younger photographers who are even now discovering vintage camera equipment.

That Lovely Yellow Glass

The above lens is an excellent Pentax Super-Takumar 1:1.4/50. I photographed it on a light table to show off its lovely yellow glass. The color comes from radioactive thorium-oxide, which some manufacturers used in some lenses through the 1940s, 50s and 60s. This “doping” increased the refractive index of the glass, reduced chromatic aberration, and allowed manufacturers to make smaller lenses. The thorium is in the glass… not on it. And over time, radioactive decay turns the glass yellow.

Such lenses are still fine for photography (and can even add a little yellow filtering, when desired). My Pentax lens is a well-known victim. Another, is the highly respected Nikkor AI 35mm 1:1.4. My copy, though, hasn’t turned yellow (yet). Both lenses also happen to be “fast” 1.4 glass, which the doping helped to make more compact. (I’ve heard that Canon’s FD 50mm f/1.2 is also radioactive, but my copy of its FL 55mm f/1.2 cousin doesn’t appear to be.)

Are they Dangerous?

The above Imaging-Resource link tries to put their radioactivity in perspective, by pointing out that most of the lenses’ rads exit safely through their fronts (which would also prevent film fogging). But this informative video warns that this may not always be true (and demonstrates that the lenses can affect digital sensors).

Both sources, though, agree that a user’s potential exposure would be fairly minimal… unless one holds a camera near their face for extended periods. Here’s how Imaging Resource quantifies possible dosage levels:

“If you pressed the front element of the lens up to your body for an hour, it’d be about the same as an x-ray. So as long as you’re not sleeping using your camera bag as a pillow, you’re probably fine.”

Other Manufacturers

Pentax, Nikon and Canon aren’t the only manufacturers to use thorium-oxide. The above video lists others. And here’s an even more extensive list of Geiger-tested lenses. It claims that Kodak was one of the greatest adopters– with thorium used in some Pony, Signet, Instamatic and Ektar lenses (especially Aero-Ektars).  (If readers know of any other manufacturers and lenses, please tell us in the comments.)

But you don’t necessarily need a Geiger Counter to determine if a vintage lens is radioactive. Just look through it for that lovely yellow color. And as Pet Clark might say if she sang about photography today: “Don’t Sleep with Yellow Lenses, Darlin’!”

–Dave Powell is a Westford, Mass., writer and avid amateur photographer.

Contribute to 35mmc for an Ad-free Experience

There are two ways to experience 35mmc without the adverts:

Paid Subscription - £2.99 per month and you'll never see an advert again! (Free 3-day trial).
Subscribe here.

Content contributor - become a part of the world’s biggest film and alternative photography community blog. All our Contributors have an ad-free experience for life.
Sign up here.

About The Author

27 thoughts on “Don’t Sleep with Yellow Lenses, Darlin’”

  1. You have to get quite close to the surface of the glass to get appreciable exposure.
    Thorium eyepiece glass would not be a good idea, as you would be putting a radiation sensitive organ close to the surface of the radioactive glass for a fair time, but I don’t think any camera eyepieces were ever made with that glass… (possibly some military sights – governments seem to have been a bit blaze with the long-term health of their soldiers sometimes).
    I had read somewhere that the yellowing/browning was due to decay into iodine salts, but I can’t find my sources, so I may have speculated that up at some time in the past.
    In any case, the yellowing/browning is a product of radioactive decay, so a brown-ish lens should (theoretically?) be less radioactive than it was at the time of manufacture.
    Supposedly if a concave lens goes yellow/brown it should be darker towards the edges… a sort of STF effect?

    1. I think it’s safe to assume that these lenses are effectively as radioactive today as they were when they were made. Almost all thorium is thorium-232 which has a half-life of about 14 billion years, so pretty much all of that that was in the lens when it was made is still there. The nastier ones might be thorium-230 which has a half-life of 75,000 years, but there’s almost none of that in thorium we use to make things I think. There are some even nastier isotopes but they also occur only in trace quantities (and the nasty ones will be pretty much gone by now).

      On the other hand, isotopes with very long half-lives are by definition not very radioactive! Indeed, I have read that the dose you might realistically expect from a lens like this (if you, for instance, carry it around with you rather than have it strapped to your eye!) is somewhat lower than the dose you might expect from living on Earth, and really quite a lot lower than the dose you would get if you live in Cornwall, or fly a lot. However I haven’t actually looked that up recently, so take this with a pinch of salt.

      1. Good point Tim… Radioactive decay is an extremely long process in “relatively” stable elements! Do you live in Cornwall? If so… lucky you. Loved Cornwall.

    2. Murray Leshner

      I prefer to call the discoloration ‘browning’ instead of yellowing, because yellow is good for contrast w/b&w film.

      Aero-Ektars (not all, but plenty) were the first application of rare-earth glass recipes and synthetic optical adhesive (acrylic instead of balsam tree source).

      Astrophysicist Michael Briggs wrote a great deal about the ‘color centers’ that develop as a form of radiation damage to the glass. It takes decades to develop and can usually be cleared with UV exposure…for another few decades. The radiation, however, remains.

      The ‘problem’ was discovered when ‘occupational’ use (some number of hours per day) of microscope ocular lenses made with such glass began to reveal a disproportionate number of eye cancers. It was eventually stopped by manufacturers.

      The addition resulted in much higher indices of refraction, allowing more radical (my term) lens designs. There is another radioactive compound I forgot the name of, which is not associated with the discoloration, and thus less obvious.

      In the two most-documented Aero-Ektar lenses, the amount of Thorium (Oxide) is more than a trace…I think I read 11% and 14% by weight for the two rearmost optical elements. I have think that was in a patent but Michael Briggs sure is not easy to find anymore. Those lenses will fog film…or capture particles for shorter term storage with film loaded.

      I have almost completely ‘cleared’ browning from two AE lenses and a 50(?)/1.4 Takumar of some type (trim ring was removed because the threads were damaged). Sunlight (if you can find it in sunny areas) can work. I left my three lenses for 27 days over a UV fluorescent lamp exposure unit.

      I keep them away from living spaces & am gradually getting rid of those I don’t see myself using…like a Kodak Signet 35 I shot one roll with and never got around to developing.

      1. Great info, Murray… Thanks! As you’ll see in a couple of my other replies, I decided to avoid the sun-exposure approach after its heat may have vaporized lubricant inside a different lens. But if a UV lamp doesn’t generate as much heat, it could be worth a try!

    3. Hi Bob and everyone… thank you for all your comments! Unfortunately, I was on vacation near Italy’s Dolomite mountains and internet access was pretty spotty up there.

      Yes, the bottom line is that while the very existence of radioactive lenses might be of interest to today’s new generation of vintage shooters, the risks of using them are agreed to be very low.

      In fact, one might face greater dangers eating off of vintage red Fiesta dinnerware! Until 1972, the Homer Laughlin Company used depleted uranium oxide to create the popular color. And if the plates, cups, saucers, and bowls were damaged in use, one might ingest very small amounts of radioactive glaze. But any red Fiestaware made after ’72 should be safe.

      As should radioactive lenses!

  2. Hi there: You mention the Nikkor 50mm f1.4 Ai. I have never heard of that lens containing a radioactive element. Could you have meant the Nikkor 35mm f1.4?

    1. Hi Gil… Thanks for pointing that out! Yes, I meant the 35mm 1.4 lens… but probably typed “50mm” because I was thinking about my radioactive 50mm Pentax lens. I’ve made that correction!

  3. Too bad the Imaging-resource link is broken due to their recent demise… You can retrieve it thanks to though :

    And if you want a bit more information you can go to with a list or such lenses, and gives plenty of comparisons and ordrers of magnitude (it’s often annuals doses though).

    For the yellow in the lens, I’ve often heard it can be mitigated by letting the lens sitting in the sun (or other UV light).

    1. Yes, bad timing with that Imaging-Resource link! Thanks for the archive URL… and the article now points to it too. Good catch NIKOJORJ!

      I did once try that sun trick with another radioactive lens. But in the sun’s heat, the lens developed an internal film that looked like tiny water droplets… but didn’t evaporate or disperse. So I’m letting my Pentax 50/1.4 stay slightly yellow!

  4. Hi: You mention the Nikkor 50mm f1.4 Ai as a lens with radioactive glass. I have never heard that before. Could you have meant the Nikkor 35mm f1.4?

    1. Hi again Gil,

      Your question seems to have appeared twice… but I just responded to the earlier one!

  5. My camera dry cabinet is 4 feet from my pillow, it contains my lenses including the Canon 50/1.4 ltm (thorium). Honestly, I’m not concerned as the effects are so minimal. I’m more concerned about not having lenses in a dry cabinet due to my region’s high humidity. Tokyo summers are brutal for humidity which is a recipe for lens fungus. Could I move the cabinet? Sure. Will I? Probably not.

    1. You are absolutely correct, John, to worry more about humidity than radiation! And even if the lenses did throw off worrisome amounts of rads, your wood cabinet would probably stop the Alpha portion. Small amounts of Gamma would still escape… but not enough to be dangerous.

  6. Don’t take a transatlantic flight and don’t live in Cornwall. Dave, I agree that users should be aware of the situation regarding these and other lenses but peoples reaction to radioactivity is often out of proportion to the risks. A transatlantic flight is equivalent to about one hundred x-rays – pilots beware? Yes they are at risk – up to 90% higher than the average population risk for melanoma – but its not something that would dissuade many from becoming a pilot. Cornwall (full of granite) has 10 times the the UK average radiation risk from radon for more than 1% of homes. And by the way, don’t buy a 1940s luminous watch – the luminence with have stopped but it’s still very radioactive. Oh dear, I’ve just calculated over 1000 muons (from cosmic rays) have passed through my brain while I was writing this. Billions in my lifetime – maybe that explains a lot …… 🙂

    1. Hi Again Ibraar,

      I just opened the video link in my article, and image color seems to be more affected than contrast. It’s in the section where “Radioactive Drew” switches back and forth between photos of his face taken with radioactive and non-radioactive lenses. I definitely see a color shift, but contrast is harder to gauge!

  7. I love Takumar lenses, shot with a Spotmatic for 10 years. When I started shooting primarily black & white it was the 1.4/50 all the way. And I would totally sleep with this lens by my side.

  8. Some versions of the 55mm ƒ/1.8 Pentax lenses also had the thorium elements. My ƒ/1.8 Super-Takumar, purchased in 1971, yellowed over time. But a mid-1970s SMC Takumar 55mm ƒ/1,8 (super multi-coated) has remained clear, so it contains different glass.

    1. Thanks Kodachromeguy! Yes, I did read somewhere that thorium-oxide doping continued a little into the ’70s… until better “ways” were found to achieve the same results.

  9. Joe Van Cleave

    I seem to remember something about exposing the yellow glass to UV light for an extended period of time will reduce the yellow coloration.

    1. Thanks Joe. I did try that once… but when I exposed a lens to the sun’s heat, a condensation appeared on internal elements surfaces. It may have been lubricant, because it’s still there two decades later. As I said in another comment, I’ll happily live with my yellow Pentax lens!

  10. Thorium releases alpha-particles which can be shielded with a simple piece of paper. The radioactive decay within the lens will hardly ever leave its surface. The particles from the surface however are dangerous when inhaled or ingested.
    So: Don’t lick your lens and don’t inhale over its surface.
    Otherwise have lots of fun with these.

    1. Thanks Dirk… Love that info! Sounds like the original “radioactive red” Fiesta dinnerware may have been pretty darned dangerous. But “photo fun” to be pursued!



Leave a Comment

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Scroll to Top