Recent omnidirectional camera designs aim a conventional camera at a mirror that expands the camera's field of view. This wide view is ideal for three-dimensional vision tasks such as motion estimation and obstacle detection, but these applications require an accurate model of the imaging process. We present a full model of the imaging process, which includes the rotation and translation between the camera and mirror, and an algorithm that determines this relative position from observations of known points in a single image. We present tests of the model and of the calibration procedure for various amounts of misalignment between the mirror and camera. These tests show that the algorithm recovers the correct relative position, and that by using the full model, accurate shape-from-motion and stereo matching are possible even if the camera and mirror are severely misaligned.