Distortion by ophthalmic lenses calculated at the farpoint sphere

Distortion by ophthalmic lenses has generally been calculated with respect to an ideal flat image surface. This results in improper distortion values as the ideal image surface for the mobile eye is the farpoint sphere. Results of distortion calculations for lenses ranging from +20 to -24.65 D are given for point focal lenses (corrected for oblique astigmatism), Percival lenses (designed to place the circle of least confusion on the farpoint sphere), minimum astigmatism and minimum distortion lens forms. Distortion of point focal Ostwalt lenses of powers +6.60 to -24.65 D range from 4.45 to -28.31% when computed at the flat image surface. Referred to the farpoint sphere the corresponding distortion values are +0.85 to -13.54%. Similarly, the distortion of the minimum astigmatic forms of lenses ranging in power from +20 to +7 D are +27.31 to 4.64% at the flat image surface but decreased to +16.38 to 0.84%, respectively, at the farpoint sphere. If the rotations of the unaided ametropic eye that are required to fixate two points in the field are compared with the necessary rotations with a spectacle correction (+20 to -24 D), it is found that the corrected eye rotation errors at the flat image surface range from 14.87 to -4.53%, but drop to +0.87 to -1.88% when referred to the farpoint sphere. It is concluded that distortion at the farpoint sphere is a more valid measure of distortion for the mobile eye than the classical measurement of distortion. This has implications for the study of space perception through lenses.