A system includes: a group of first light sources arranged around a central axis, the group being separated from the axis by a radial distance defining an aperture in the group; a plurality of second light sources; a detector array; and an optical system adapted to provide light from the second light sources through the aperture to a cornea of an eye, and to provide images of the first light sources and images of the second light sources from the cornea, through the aperture, to the detector array. The optical system includes a field flattener such that a shape of a surface of best focus on the detector array curves away from detector array in generally a same direction as the eye and a sclera of the eye.