Principles and designs of the iseikonic lenses for near vision

The purposes of this study were to find the problems of afocal iseikonic lenses as used for near vision and to elaborate the principles and designs of the ideal near iseikonic lenses.

By analyzing the image-object relationship and imaging processes, the formulae were derived to quantitatively and qualitatively describe two errors of afocal iseikonic lenses as used for near vision: 1) inequality of accommodative demands to both eyes and 2) the difference between the actual and the nominal magnification. Formulae were derived to calculate and design the ideal near iseikonic lenses.

For iseikonic lenses now used for near vision, inequality of accommodative demands to both eyes and difference between the actual and nominal magnification increased as the magnification increased and the viewing distance decreased. Afocal iseikonic lenses have significant inequality of accommodative demands to both eyes with 3% magnification at a viewing distance of 25 cm or with 5% magnification at 40 cm. The ideal near iseikonic lenses were found to be flatter than the afocal ones. A series of the ideal near iseikonic lenses were designed with some practical considerations. The central thickness and the refractive index of the lenses affect magnification and lens shape. The thicker the lenses, the higher the magnification. The higher refractive index also gives higher magnification.

Iseikonic lens design is much different for distance and near vision. When the iseikonic lenses are used at a distance other than the designated distance, they will cause blurred vision and artificial aniseikonia as a result of inequality of accommodative demands to both eyes. In research as well as clinical settings, it is necessary to apply the ideal near iseikonic lenses for near vision.