Comparison of the depths of focus with the naked eye and with three types of presbyopic contact lens correction

Experiments are described in which monocular, high-contrast, letter acuity was measured as a function of target vergence (effective distance) for six simulated presbyopic subjects, who were placed under cycloplegia with a 4mm diameter artificial pupil. Measurements were made with the naked eye or single-vision spectacle lens corrected eye, and with three types of simultaneous-vision (simultaneous image) ‡ presbyopic contact lens correction: a centre-near biofocal; a centre-distance varifocal or progressive power lens; and a centre-mear varifocal. Results show that all designs usefully improve the range of distances over which an adequate standard of vision is achieved, the depth of focus typically being increased by about 1–2 dioptres, depending upon the criterion used to define depth of focus. Peak visual acuity at optimal focus is, however, slightly worse with the presbyopic contact lens corrections than with the naked or spectacle-lens corrected eye. Intersubject differences suggest that individual lens characteristics, the aberrations of the individual eye and lens centration may all play a role in determining the optical effectiveness of any particular design.