  In Binder's approach the reduced interface tension sigma of the Ising model
in the broken phase is determined from the finite volume effects of the
partition function Z(M) at fixed total magnetization M. For small |M| the
partition function of a system of size L^d with periodic boundary conditions is
dominated by configurations with two interfaces, such that Z(M) ~ exp(- 2 sigma
L^{d-1}). Capillary wave fluctuations of the interfaces correct this result to
Z(M) ~ exp(- 2 sigma L^{d-1}) with x = -1. The knowledge of the pre-exponential
behavior allows an improved fit of numerical data, and a determination of the
interface stiffness.
