Analytical expressions are derived for the power spectral density of the current in optical heterodyning when phase front curvature mismatch, tilt or axis displacement occurs between the beating light beams. The signal beam is the far field of an incoherent source of circular shape, and the local oscillator is the fundamental mode of a stabilized laser. The cases in which the signal beam either illuminates the detector coherently or has a coherence area small compared with the detector area are shown to have simple solutions. An analogous analysis is carried out for homodyning (self-beating). The dependence of the signal-to-noise ratio on the detector aperture and optimum and tolerance values for the latter are obtained for a given misalignment.