Fig. 2. Schematic light paths in a superposition eye (modified from Land, 1984). (A) Focal point at the retinal surface. r is the radius of curvature of the distal surface of the rhabdom layer, w is the clear zone width, i.e. the distance of the rhabdom layer surface to the proximal tips of the crystalline cones, and c is the thickness of the crystalline cone and facet lens layer. R=r+w+c is the radius of the corneal outer surface, or the eye radius, and p=r+w is the radius of curvature of the proximal cone tips. is the angle of incidence of a ray through the vertex of a facet, and the angle of that ray with the ommatidial axis after having passed the facet lens and crystalline cone is the exit angle ß. The central ray is defined by =ß=0. The oblique ray travels a distance q in the clear zone before intersecting the central ray at point P, which ideally coincides with the tip of the central rhabdom. (B) Focal point proximal to the retinal surface. The focal point P* is then located at a distance r* from the center of curvature of the eye and w* from the proximal cone tip. The distance traveled across the clear zone by the oblique ray is q*. The difference in optical path lengths of the two rays in point P* is u+nq*–nw*=u+n(q*–w*), where u is the path length difference between the ray with incident angle, , and the central ray, when reaching the corneal surface, and n is the refractive index of the eye tissue.