Are your old lenses radioactive?


posted Monday, May 6, 2013 at 12:59 PM EDT

A radioactive Leica 50mm f/2

For nearly 30 years, camera producers were making lenses that were optically excellent but had a very curious side effect — they were notably radioactive. These so called "radioactive lenses" put off enough radiation that to this day they set off a geiger counter (but don't worry, the levels are so low as to be near harmless).

If you take a lens made between 1940 and 1970, crafted by luminaries such as Carl Zeiss, Kodak, Olympus, Voigtlander and more, and hold it up to a geiger counter, you'll get a measurable reading — particularly from the front element. But why are these lenses radioactive? It turns out that for decades, it was standard practice to include thorium oxide in the lenses. This inclusion made the glass highly refractive, but with low dispersion, allowing for dramatic re-angling of light without losing optical quality. Unfortunately, the downside is that thorium oxide is radioactive.

Thankfully, the radiation levels are quite low, especially out of the back element (because if they weren't, the radiation could fog the film). They generally output 10 mR/hr if you're within three feet — so 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.

Keep in mind there was a time when a great number of everyday objects contained radioactive materials: cookware, radium and tritium watches, even smoke detectors. While some of these were dangerous, some weren't. Radioactive doesn't automatically mean deadly, or else we'd all be doing a lot less flying!

If you're curious if your lens might be radioactive, there's a complete list at Camerapedia.

(via Reddit)