The interferogram produced by wide-aperture interferometers is studied both theoretically and experimentally. The fringe spacing is shown to increase non linearly with the numerical aperture and the fringe contrast to decrease with the optical path difference. Phase measurements with wide-aperture interferometers therefore require calibration and the phase can be measured only over a limited range. A calibration is given for accurate phase measurements and the range over which the phase can be measured is calculated. Experimental measurements are presented and compared with theory.