Use of annular artificial pupil in the determination of modulation transfer function of the eye

According to Ref. [1] primary spherical aberration of the schematic eye would be completely absorbed by an annular aperture when its obstruction ratio epsilon is greater than or equal to 0.60. Consequently the contrast sensitivity function, Tcs, determined with the use of such an annulus, Tcs (A), is the MTF of the retina-brain system, Trb, weighted by the MTF of the annulus, T. . Trb thus obtained can be used to determine the MTF of the eye, Te. Annular apertures of various sizes and obstruction ratios used for the determination of Te have been analysed. It is shown that there are several factors other than incomplete absorption of primary spherical aberration that would prevent the use of annular pupil to derive Trb under unrestrained conditions. This is due to the fact that the apparent T.'s of annuli used in the actual determination of Tcs(A) usually deviate from the theoretical T.'s for the passive eye to different extents. However, it is found that the T. for annulus of o.d. = 2.7 mm and epsilon = 0.60 conforms more or less to the theoretical T. and can be used to derive reasonably accurate Te(2.7). It is also possible to estimate Te(less than 2.7) from Te(2.7) by some empirical correction.