Moire fringe projection techniques are gaining popularity due to their non-contact nature and high accuracy in measuring the surface shape of many objects. The fringe patterns seen when using these instruments are similar to the patterns seen in traditional interferometry but differ in that the spacing between consecutive fringes in traditional interferometry is constant and equal to the wavelength of the source. In moire fringe projection, the spacing (equivalent wavelength) between consecutive fringes may not be constant over the field of view and it depends on the geometry (divergent or parallel) of the set-up. This variation in the equivalent wavelength causes the surface height measurements to be inaccurate. This paper looks at the aberrations that are caused by this varying equivalent wavelength and a calibration process to determine the equivalent wavelength map.