  We discuss the effect of the external fermion masses in the flavor-changing
radiative transitions of a heavy fermion (quark or lepton) to a lighter fermion
at the one-loop level, and point out an often overlooked crucial difference in
the sign of a charge factor between transitions of the down type $s\to d\gamma$
and the up type $c\to u\gamma$. We give formulas for the $F\to f\gamma$
effective vertex in various approximations and the exact formula for $t\to
c\gamma$ and $\tau \to \mu \gamma$.
