The Leica forum on Photo.net is a hoot. Although there are a number of retired dentist equipment fondlers who bore me to tears, there are also quite a few fun, smart and generally helpful people.

Today there was a discussion about why the glass used to make Leica lenses is (it is said) superior to most others. One response stated, in all seriousness, that it was “Special glass made with rare earths from the Carpathian Mountains.” Another post chimed in and claimed that it couldn’t be, since “…rare earth elements are often radioactive.”

Then, almost immediately, this…

“FYI this statement is basically wrong. There is one naturally occuring isotope of lanthanum (La-138) that has a 110 billion year half-life which makes it essentially not radioactive for all but the most sensitive physics experiments. Its half-life is 100x longer than potassium-40, which we ingest in large amounts whenever we eat sources of food rich in potassium such as bananas. I have personally measured radioactivity in bananas and it is detectable.

Radioactive lenses contain thorium which is an actinide, not a rare earth (defined as lanthanides + scandium & yttrium). Thorium decays in a long cascade before reaching a stable isotope of lead. This chain results in a multitude of radioactive emissions including energetic gamma rays.”

Did you catch that? “I have personally measured radioactivity in bananas and it is detectable.”

How can a fella not want to read every single thread, when there are people like that about?