  The determination of renormalization factors is of crucial importance. They
relate the observables obtained on finite, discrete lattices to their measured
counterparts in the continuum in a suitable renormalization scheme. Therefore,
they have to be computed as precisely as possible. A widely used approach is
the nonperturbative Rome-Southampton method. It requires, however, a careful
treatment of lattice artifacts. They are always present because simulations are
done at lattice spacings $a$ and momenta $p$ with $ap$ not necessarily small.
In this paper we try to suppress these artifacts by subtraction of one-loop
contributions in lattice perturbation theory. We compare results obtained from
a complete one-loop subtraction with those calculated for a subtraction of
$O(a^2)$.
