How to use a thinner pen?

It seems that I don't manage to use a pen thinner than the default 0.5bp.
When I give the command: `pickup pencircle scaled 0.1bp', or even `pickup
pencircle scaled 0.0000001bp', my eye can't notice any difference with the
default thickness.  Instead, a bigger thickness works fine.  What am I
missing?

Thanks for any help.

Rodolfo

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