  The ensemble-switch method for computing wall excess free energies of
condensed matter is extended to estimate the interface free energies between
coexisting phases very accurately. By this method, system geometries with
linear dimensions $L$ parallel and $L_z$ perpendicular to the interface with
various boundary conditions in the canonical or grandcanonical ensemble can be
studied. Using two- and three-dimensional Ising models, the nature of the
occurring logarithmic finite size corrections is studied. It is found crucial
to include interfacial fluctuations due to "domain breathing".
