We examine the properties of the transverse eigenmodes of optical resonators containing a hard aperture. We show that for orthogonal optics and arbitrary aperture shape, the trace of the ray matrix and the scaled aperture size determine the major properties of interest: loss, frequency, and distortion that is due to diffraction. We discuss three different methods of reducing the Huygen–Fresnel integral equation to a matrix equation whose eigenvalues and eigenvectors can be easily found. The dependence of loss, frequency, distortion, and required matrix order on cavity parameters is presented for cylindrically symmetrical resonators. We show that undiffracted modes inadequately approximate the actual modes in nearly unstable cavities.