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According to Bef. [1] primary spherical aberration of the schematic eye would be com-pletely absorbed by an annular aperture when its obstruetion ratio ∈≥0.60. Consequently thecontrast sensitivity function, T_(cs), determined with the use of such an annulus, T_(cs) (A), is theMTF of the retina-brain system, T_(rh), weighted by the MTF of the annulus, T_⊙.T_(rh) thus obtain-ed can be used to determine the MTF of the eye, T_e. Annular apertures of various sizes andobstruction ratios used for the determination of T_e have been analysed. It is shown that thereare several factors other than incomplete absorption of primary spherical aberration thatwould prevent the use of annular pupil to derive T_(rb) under unrestrained conditions. This is dueto the fact that the apparent T_⊙’s of annuli used in the actual determination of T_(cs)(A) usual-ly deviate from the theoretical T_⊙’s for the passive eye to different extents. However, it isfound that the T_⊙ for annulus of o.d.=2.7 mm and ∈= 0.60 conforms more or less to thetheoretical T_⊙ and can be used to derive reasonably accurate T_e(2.7). It is also possibleto estimate T_e(>2.7) from T_e(2.7) by some empirical correction.
According to Bef. [1] primary spherical aberration of the schematic eye would be com-pletely absorbed by an annular aperture when its obstruetion ratio ∈> = 0.60. , T_ (cs) (A), is the MTF of the retina-brain system, T_ (rh), weighted by the MTF of the annulus, T_⊙.T_ (rh) obtained-ed can be used to determine the MTF of the eye, T_e. Annular apertures of various sizes and construction ratios used for the determination of T_e have been analyzed. 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 T_ (rb) under unrestrained conditions. This is dueto the fact that the apparent T_⊙’s of annuli used in the actual determination of T_ (cs) (A) usual-ly deviate from the theoretical T_⊙’s for the passive eye to different extents. However, it isfound that the T_⊙ for annulus of od = 2.7 mm a nd ∈ = 0.60 conforms more or less to the theoretical T_⊙ and can be used to derive reasonably accurate T_e (2.7). It is also possibleto estimate T_e (> 2.7) from T_e (2.7) by some empirical correction.