The Radioactive Lenses in Your Camera Bag
Point a Geiger counter at the rear element of a Super-Multi-Coated Takumar 50mm f/1.4 and it will start clicking. That is not contamination and it is not an accident. The radioactivity was designed in, at the factory, by engineers who knew exactly what they were doing, and for about thirty years it was one of the quiet secrets behind some of the sharpest consumer optics ever sold.
Why anyone would put thorium in a lens
Lens design is a war against physics. Every element you add to fix one aberration introduces others, and every exotic correction adds cost. What designers wanted was glass with a high refractive index and low dispersion at the same time, which lets a simple design bend light hard without splitting its colors apart.
Thorium dioxide does exactly that. Its optical behavior resembles fluorite, the miracle crystal, but it could be melted into ordinary glass batches at up to roughly 30 percent by weight. Kodak patented practical thoriated glass formulas in 1949, and from the 1940s through the 1970s it went into an enormous amount of serious optics. The trade-off is the one you already guessed: thorium has no stable isotopes. Every atom of it is radioactive, with a half-life measured in billions of years.
Who used it
The most prolific user was Kodak itself, which put thoriated glass in everything from professional Ektars down to high-end Instamatics. The most famous examples are the wartime Aero-Ektars, built in quantity for aerial reconnaissance and hot enough to be a standing curiosity among collectors ever since.
On the M42 mount, the story runs straight through the files on this site. The 7-element Super-Takumar 50mm f/1.4 and its Super-Multi-Coated successor carry thoriated rear groups, which is why the counter clicks hardest at the back of the lens. The SMC Takumar 85mm f/1.8 is on the standard references' thoriated list too. Carl Zeiss Jena used it in the zebra-trim Pancolar 50mm f/1.8, and only the zebras: the later multicoated versions are clean, which makes the yellow cast itself a dating tool. Fuji's early 50mm f/1.4, Olympus and Canon fast fifties of the era, and a long tail of others round out the club. Manufacturers reserved the good radioactive glass for their fastest, proudest designs, so the list of thoriated lenses reads suspiciously like a list of the era's best lenses.
Why the glass turns yellow
A thoriated lens irradiates itself, from the inside, for decades. That steady internal bombardment knocks defects into the glass structure called F-centers, and F-centers absorb blue light. The result is the famous amber-brown cast that collectors either curse or use as a bargaining chip.
The fix is almost absurdly simple: light. Strong UV exposure heals the F-centers, and a week or two under indirect sunlight or a UV lamp clears most of the cast. The radioactivity stays, because the thorium is going nowhere on a human timescale, but the glass runs clear again. A deeply yellowed Takumar priced down for its color is one of the quiet bargains of the vintage market: the defect is real, visible, and reversible.
The part everyone actually wants to know
The dose is small. The radiation is mostly alpha, which your skin stops. A physics study at Sweden's Royal Institute of Technology worked out the exposure for a professional photographer using a typical thoriated lens and landed at about 0.2 percent of the yearly allowable eye dose and 0.17 percent for the whole body, under conservative Swedish standards. You receive more from cosmic rays on a long airline flight than from an afternoon shooting a Takumar.
The exceptions are instructive rather than scary. Thoriated glass in an eyepiece pressed against your eye is genuinely bad, because the cornea has no skin to protect it; regulators barred that use, and radioactive eyepieces are rare. And in the film era, a hot rear element could slowly fog film left loaded in the camera for months, which says more about the radiation's patience than its power. Shoot the lens, store it on a shelf like any other, and do not grind it into powder and inhale it. That is the entire safety briefing.
Why it stopped
By the end of the 1970s the practice was dead. Worker safety at the glass plants, tightening regulation, public unease about all things radioactive, and the arrival of lanthanum glasses that delivered similar optics without the decay chain all pushed the same direction. Nobody has put thorium in a consumer lens since, which means every thoriated lens is a closed catalog: the supply is fixed, the story is finished, and the survivors are all there will ever be.
What the file records
Every reference on this site that is known or reported thoriated carries a thorium box on its datasheet saying exactly that, in plain language. It is one of this hobby's best stories: the era when the best glass in the world came with its own faint tick, and the yellowing that scares off half the buyers can be cured with a sunny windowsill.