Radioactive lens effects seen on film?

I have a Super Takumar 50mm f/1.4, one of the more common lenses with a thoriated rear element. I’ve owned it for forty years, and a couple times left 400 speed film in the camera for several months with the lens mounted, and never seen any effect.


First, between the lens and the film is a mirror and shutter curtain (in an SLR — in a Speed Graphic with an Aero Ektar, there’s more distance, as well as a dark slide in almost all cases).

Second, because the primary radiation from decaying thorium is alpha particles (helium nuclei stripped of their electrons); they have very little penetrating power (in most cases, a few inches of air or a single sheet of paper will stop nearly all of them, never mind the metal reflective coating and glass of an SLR mirror). Virtually no alpha particles will penetrate metal parts of the camera body or lens body. Therefore the only radiation that could expose the film must pass through the reflex mirror, its mount plate (usually thin metal), and the shutter curtain (either opaque cloth as in my Spotmatic SP or metal blades as in my Ricoh Singlex II) — and again, that isn’t going to happen with alpha, or not enough of it to matter.

Now, most of these radioactive lenses are fifty-some years old (some as much as three decades older than that), so of course they have other decay products mixed with the thorium in the thoriated element(s), but most natural decay paths still produce primarily alpha radiation, with occasional beta (loose positrons, which produce gamma photons when they annihilate with an electron) and almost never direct gamma emission; beta penetrates more than alpha, but so little is produced that it won’t expose the film in a reasonable time, nor (apparently) will the gamma produced by beta annihilations.

So, bottom line, even several months with the lens mounted on an SLR and 400 speed film loaded in the camera produces no noticeable fogging, at least in my experience since the early 1980s. The answer might be different if the camera shutter was locked open and mirror up (with lens capped, I presume), but that would be very, very unusual situation.