  Last years, there has been a renewed interest in the utilization of
statistical field theory methods for the description of systems at equilibrium
both in the vicinity and away from critical points, in particular in the field
of liquid state physics. These works deal in general with homogeneous systems,
although recently the study of liquids in the vicinity of hard walls has also
been considered in this way. On the other hand, effective Hamiltonian
pertaining to the $\phi^4$ theory family have been written and extensively used
for the description of inhomogeneous systems either at the simple interface
between equilibrium phases or for the description of wetting. In the present
work, we focus on a field theoretical description of the liquid vapor interface
of simple fluids. We start from the representation of the grand partition
function obtained from the Hubbard-Stratonovich transform leading to an exact
formulation of the problem, namely neither introducing an effective Hamiltonian
nor associating the field to the one-body density of the liquid. Using as a
reference system the hard sphere fluid and imposing the coexistence condition,
the expansion of the Hamiltonian obtained yields a usual $\phi^4$ theory
without unknown parameter. An important point is that the so-called capillary
wave theory appears as an approximation of the one-loop theory in the
functional expansion of the Hamiltonian, without any need to an underlying
phenomenology.
