In displacement measurements by two-beam interferometers, the wavefront curvature of a laser beam causes a systematic increase of the fringe period. This increase depends on beam collimation: It is null for a plane wave and proportional to the squared divergence of the beam. With interfering beams not perfectly recombined, an additional fringe-period error is caused, with the effect of counteracting and also of compensating for and prevailing over the usual error. We describe this hitherto unsuspected effect and give a correction equation. (c) 2006 Optical Society of America.