This paper concerns a numerical technique and associated modeling results for evaluating coherent phase transfer function (CPTF) degradation that is induced by wave aberrations of the imaging train of a laser Fizeau interferometer. The CPTF is defined as the ratio of the Fourier transform of the phase portion of the complex image of a phase object formed by the interferometer to the Fourier transform of the phase object itself. A unit step function is used as the object. The aberrated image is calculated using an optical design program. The traditional instrument transfer function is the amplitude portion of the CPTF. It has been found that the phase portion of the CPTF is needed for properly characterizing the frequency response of the system when a significant amount of coma is involved. Based on the results of the study, an aberration tolerance criterion is proposed for the optical design of a laser Fizeau interferometer.