Analytic approximation of the off-axis modulation transfer function of the eye.

Published experimental measurements of the ocular modulation transfer function (MTF) in the peripheral field are approximated by the expression T(f) = exp[-(f/fc)n], where T(f), f, fc, and n are modulation transfer, spatial frequency, spatial frequency constant and MTF index, respectively. It is shown that n (which describes the shape of the MTF) remains relatively constant at about 0.9 for field angles out to 40 deg but fc (which defines the spatial frequency scaling) declines steeply over this range, depending upon the pupil diameter and conditions of focus. The oblique astigmatism of the eye plays a major role in the off-axis changes in fc at field angles > or = 20 deg. The approximation may be useful in allowing the form of the degraded optical stimulus in studies of the peripheral retinal function to be evaluated.