They're not seeing any alphas because the radioactive element on the 50/1.4
is an internal one. Thorium and other daughters are alpha emitters, but
they're blocked by the other elements. I presume the difference between the
front and back is due to the metal lens mount, and maybe more glass,
blocking more betas, and not very many gammas are emitted.
Not only did we evolve in a sea of low-dose radiation, we still live in it
so it's not possible to get zero background radiation. It only makes sense
to be careful with things that are known to be radioactive, but normal use
of the 55/1.2 isn't going to hurt anyone.
Mark
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