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I'm also asking a similar question on Physics SE.

As we know, some old lenses used Thoriated glass which give off non-negligible amounts of radiation. Although the radiation isn't really harmful to the photographer, I'm wondering if it could be detected on film photographs. The radiation that hits the photographer would have had to travel through the film to get to him/her. (Nothing is, however, coming out of the eyepiece since the radioactive radiation doesn't usually reflect off the mirrors like light.)

For this SE, I'm wondering if anyone knows of an example photo where such radiation effects is visible (presumably as black specks on the negative). Perhaps someone has mistakenly wound past a frame without exposing it, and found specks on that unexposed frame? I'm guessing that it's most easily seen on some high-sensitivity and/or large-grain B&W film.

Maybe such effects can be picked up by a digital sensor, but I'm guessing that it could be hard to distinguish that from noise.

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  • Side-note: The app "RadioactivityCounter" allows you to measure radioactivity levels using a smartphone-camera and a bit of black masking tape, by counting the pixels that light up due to background radiation. Sadly, the app has not been updated in a while (at least on iOS, latest supported Apple device is iPhone6s), but there has been a recent nature paper: nature.com/articles/s41598-021-92195-y Sep 16 at 16:49
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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.

Why?

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 electrons potentially able to emit x-ray on impact with metals, but usually not energetic enough for that) 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.

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.

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    Re, "metal parts...reflex mirror...shutter curtain" Most of the alpha particles won't even escape from the lens element itself. Only those emitted from very close to the surface of the glass will be able to make it out. Sep 15 at 17:04
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    In addition, only those particles that are stopped by the emulsion will alter it. Those that are penetrating enough to go through mirror+shutter will likely go through the emlsion as well.
    – xenoid
    Sep 15 at 17:09
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    @xenoid While true, don't forget that film emulsion has a significant amount of silver in it, which like most other metals has a higher interaction profile (hence why better shielding per thickness for something like gamma) than organics, water, etc.
    – Zeiss Ikon
    Sep 15 at 17:46
  • Minor point: regular beta decay releases electrons, not positrons. Beta+ is less common, and it looks like the decay chain of both thorium isotopes which occur naturally in appreciable amounts (230 and 232) only have regular beta decays in them
    – llama
    Sep 16 at 17:00
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    There is certainly SOME gamma radiation emitted - cheap geiger counters will respond decisively to radioactive lenses (not the kind with trace thorium in the glass really - but certainly those with INTENTIONALLY thoriated glass), and these are usually not equipped to measure alpha/beta radiation. Sep 16 at 19:53

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