Image intensifier distortion due to photocathode curvature and electron optics is shown to be approximated by a simple two parameter odd-power polynomial. The accuracy of this fit was found to be far better than that of two other one parameter characterizations of distortion when applied to experimental data from four different model image intensifiers ranging in diameter from 9 to 14 in. The standard errors of the two parameters fits were less than 0.1 mm or 0.03% of the field of the IIs and were within the estimated measurement error.