Some of the best-loved vintage lenses contain glass made with thorium oxide. The Super-Takumar 50mm f/1.4 is the famous one, and sellers often put the word radioactive in the title as a selling point. For a buyer, the practical issues are a yellow tint, some lost light, and knowing which version you're actually looking at. This guide covers why the glass was used, which lenses the sources name, how to clear the yellowing, and what radiation experts have written about it.
Why lens makers used thorium
Designers wanted glass that bends light strongly without spreading colors apart. Camera-wiki explains that thorium oxide gives glass high refractive power with low dispersion, which lets a designer reduce chromatic aberration and use flatter, cheaper-to-make curves. Such glass could contain up to 30 percent thorium oxide by weight. The Health Physics Society's answer on the subject puts it plainly. The thorium was added for its optical effect, and the fact that it's radioactive was beside the point.
The same answer says thoriated camera lenses seem to have stopped being made in the late 1980s. Kodak was the biggest user, and Japanese and European makers used it too.
Which Takumars have it
Camera-wiki's Takumar history says Asahi used thorium oxide in several Super-Takumar, Super-Multi-Coated and a few SMC Takumar lenses between 1964 and 1973, usually in a single element at the front or back. The ones it names are the Super and Super-Multi-Coated Takumar 50mm f/1.4 in its 7-element version, the 20mm f/4.5, the 35mm f/2, the 55mm f/1.8, the 55mm f/2, the 85mm f/1.8 and the Super Takumar 6x7 105mm f/2.4.
The 50mm f/1.4 is where people get confused. The first Super-Takumar 50mm f/1.4 of 1964 had 8 elements in 6 groups. In 1965 Asahi replaced it with a 7-element design that used thorium oxide glass in the rear element, and that design carried on into the Super-Multi-Coated and SMC Takumar versions. The Oak Ridge Associated Universities museum lists the SMC Takumar 50mm f/1.4 among lenses known to contain thorium.
Telling the two Super-Takumars apart from photos is possible. According to the Takumar field guide, the first 7-element lens has the product number 37800 inscribed on the back of its aperture switch, later versions carry their own numbers such as 37801, and the 8-element lens has no number there. Camera-wiki adds that the 8-element lens's rear element sticks out further, and that on the 7-element lens the infrared focus mark moved from the right of the f/4 mark to the left. Serial numbers don't settle it, because the field guide found the two ranges overlap.
The 55mm f/1.8 also appears on camera-wiki's list. All of these are M42 screw-mount lenses from Pentax's maker, Asahi.
Other lenses on the lists
Camera-wiki keeps a longer list of lenses reported as radioactive. From brands in our catalog it includes the early-1970s versions of the Canon FD 35mm f/2, the Canon FD 55mm f/1.2, the Konica Hexanon AR 57mm f/1.2, the Olympus G.Zuiko Auto-S 50mm f/1.4, an early Nikkor 35mm f/1.4 and the Carl Zeiss Jena Flektogon 35mm f/2.8. That Flektogon is the f/2.8, a different lens from the f/2.4.
The Jena story is well documented. Marco Kröger's history of the Zeiss Jena factory says the first Pancolar 50mm f/1.8, made from 1965 to 1970, used a heavy crown glass that needed thorium dioxide in four of its six elements, and that it yellowed strongly with age. That version only came in the zebra style, with a shorter barrel than its successor. Our guide to dating Zeiss Jena lenses covers how to tell the versions apart.
Treat all of these lists as reports gathered from owners and collectors. Several lenses changed glass between versions, so the model name alone doesn't tell you which you have.
Why the glass turns yellow, and how to clear it
Wikipedia's article on thoriated glass says the browning comes from tiny defects in the glass that build up as the thorium decays. Over time the tint cuts the light getting through and pushes colors warm. Camera-wiki puts the loss at around half a stop to a stop, and its Takumar page gives a wider range, up to about one and a half stops.
The good news is that light reverses it. Camera-wiki says owners have cleared yellowed lenses with sunlight, UV lamps and bright LED lamps, and that a sunny window sill takes several days. The Takumar field guide describes leaving the lens upside down in a plastic container in a sunny window until the tint is gone. Because the cause is still in the glass, you can expect the tint to creep back slowly over the years.
If you shoot a lens that's still yellow, a custom white balance on a digital body takes out most of the cast. The lost light doesn't come back that way.
What the sources say about the radiation
We don't give safety advice. Here's what the sources we checked say, in their own words where it matters.
Camera-wiki states that typical radiation levels can approach 1 mR/hr as measured at the lens element's surface, and that at a distance of 3 ft. (0.9 m) the radiation level is difficult to detect over typical background levels. It also cites US Army research showing that glass absorbs the alpha radiation within a very thin surface layer.
In a 2001 answer for the Health Physics Society, health physicist Paul Frame wrote that the dose rate near the viewfinder of his own Pentax with a Super Takumar lens was roughly 100 microrad per hour, approximately ten times background. He quoted NUREG-1717, a report by the US nuclear regulator, which estimated an effective dose of 0.7 mrem per year for people using cameras with thoriated lenses, and called that substantially less than 1% of the 300 mrem that we receive each year due to our exposure to background radiation. He wrote that a thoriated eyepiece, which is rare, is a much bigger concern than a camera lens. The Oak Ridge Associated Universities museum page covers the same report in more detail if you want to read further.
Buying one
- Know which version you want. For the Super-Takumar 50mm f/1.4, ask for a photo of the back of the aperture switch. A five-digit number there, such as 37800, means the 7-element lens.
- Look at the color. Ask for a photo through the lens against white paper. A yellow or brown tint is the thorium element, and you can clear it at home.
- Check the rest as usual. Haze, fungus and oily blades matter more than the tint. Our guide on how to check a used vintage lens covers them.
A yellowed lens isn't a damaged lens, since a few sunny days usually clear it. What you can't fix that easily is everything else, so judge the glass first. You can see current listings on each lens page, or browse the whole catalog.
