On the compensation of horizontal coma aberrations in young human eyes.

The nature of the compensation of horizontal coma (Z(3)(1)) between optical elements of the human eye has been studied and the compensative mechanism has been attributed to a passive process linked to angle kappa of the eye. We measured the horizontal coma in the anterior cornea, the whole eye and the internal optics for 221 young subjects. Thirty-three eyes with minimum angle lambda and 53 eyes with relatively large angle lambda were selected from these eyes to test the hypothesis that horizontal coma compensation is linked to angle kappa. Significant horizontal coma in the anterior cornea was observed for the group with minimum angle lambda in both the right (-0.12 +/- 0.07 microm) and left eyes (0.12 +/- 0.10 microm), and this was well compensated by the internal optics so that the level of horizontal coma in the whole eye over a 6-mm pupil size was very low (-0.05 +/- 0.07 microm for OD and 0.02 +/- 0.08 microm for OS). The horizontal coma in the anterior cornea was significantly correlated to the horizontal coma in the internal optics for both the right and the left eye. The results suggest that there is another source of horizontal coma, in addition to that linked to angle kappa, in the anterior cornea, and also a new compensative mechanism to balance the corneal coma, perhaps in the posterior cornea or the lens.