Schematic view of the light paths. Light from the taking lens is collected by the reflective focus screen, passed through a small aperture stop, and forms a real image of the scene to the right of the concave mirror. Because the concave mirror is made aspheric to control distortion, that image is tilted toward the eye lens. The eyelens shape is free-form to correct the field tilt, but the two aspheres together introduce coma. 05