A quantitative statistical evaluation of peripheral astigmatism in the horizontal meridian of emmetropic human eyes has been made, using the measurements of Rempt <i>et al.</i> on 363 subjects. A hypothesis of Ferree, Rand, and Hardy, according to which human eyes fall into two distinct classes with respect to Sturm’s interval, is not supported by the present material. The mean values of Sturm’s interval at 20°, 40°, and 60° eccentricity can be well approximated by a symmetrical curve whose axis forms an angle of 4° with the fixation axis and therefore is close to the optical axis of the eye as derived from the Purkinje images. Theoretical values for Sturm’s interval calculated from a relatively simple eye model with aspherics, based on Gullstrand’s model, though lying within the range of experimental findings, do not fit well Rempt’s average values.
W. Lotmar and T. Lotmar, "Peripheral astigmatism in the human eye: Experimental data and theoretical model predictions," J. Opt. Soc. Am. 64, 510-513 (1974)