Fig. 9. Schematic drawing of a lens in a frame. OC is the optical center, and OA is the optic axis, which should pass through the center of rotation (CR) of the eye. Ray bundles are traced to the far-point sphere (FPS) for calculation of lens aberrations. Errors are referred to the reference sphere (RS). Stop distance, or center of rotation distance, is the sum of the sighting center distance (SCD) and the vertex distance (V).