The variation in sphero-cylindrical refractive error with position in the human eye pupil is derived from the wavefront error of two subjects. The modulation transfer function is also calculated for 1, 2 and 3 mm pupils decentred 1 and 2 mm nasally and temporally. Whilst decentration causes relatively little difference in the M.T.F. at the smaller pupil size, it can produce marked degradation for the larger pupils which can be of significance in the experimental determination of contrast sensitivity or other visual functions.