Fabry-Perot interferometers have played an important role in the conception and realization of optical masers. The authors have previously made a study of the idealized interferometer. In this paper they present some results of a continued study of the effects of certain simple forms of aberration. The first is represented by tilted plane mirrors and the second by curved mirrors. Tilting the mirror causes the mode patterns to become asymmetric and the diffraction loss to become greater. It also tends to equalize the losses of the two lowest-order modes thus causing a beating phenomenon which has been observed experimentally. In the case of interferometers with mirrors of arbitrary radii of curvature, there exist regions of low loss and high loss as the mirror spacing is varied. The loss function can be represented by a contour map of a three-dimensional model which has certain symmetry properties. This model is useful in choosing proper mirror spacing for low loss operation of optical masers.