An apparatus for determining a position of a subject's cornea, comprising a camera, a first illumination apparatus adapted to provide a first beam of light and a first diffuse portion of light that are coincident, and a second illumination apparatus adapted to provide a second beam of light and a second diffuse portion of light, that are coincident. In some embodiments, the first, second, third and fourth images are disposed on the midpoints of a square, when a cornea is located along an optical axis of camera.