  We review the dynamical equivalence between $f(R)$ gravity in the metric
formalism and scalar-tensor gravity, and use this equivalence to deduce the
post-Newtonian parameters $\gamma$ and $\beta$ for a $f(R)$ theory, obtaining a
result that is different with respect to that known in the literature. Then, we
obtain explicit expressions of these paremeters in terms of the mass of the
scalar field (or, differently speaking, the mass of the additional scalar
degree of freedom associated to a $f(R)$ theory) which can be used to constrain
$f(R)$ gravity by means of current observations.
