OPTICAL-PERFORMANCE OF ASPHERIC CONCAVE OPHTHALMIC LENSES - THE EFFECT OF VERTEX DISTANCE

Variations of power errors in the periphery of concave aspheric lenses were assessed as a function of vertex distance, Comparisons were made between the performance of different types of spherical lenses and th at of aspheric lenses (-6.00 and -8.00 D lenses were used). Off-axis m easurements were made by means of a modified focimeter, which enabled rotation of the lens around the center of the vertex: sphere. Reductio n of the vertex sphere's radius from 27 to 23 mm simulated a 4-mm decr ease in vertex distance, Results of the measurements are compared with a computed model, These results indicate that off-axis performance is more affected by a reduction of vertex distance in the case of aspher ic lenses compared to spherical lenses, Resulting increases in oblique astigmatism (OA) and mean oblique error (MOE) ape at the limit of cli nical acceptance, The importance, for the practitioner, of respecting the vertex distance selected for a specific aspheric design when fitti ng this type of lens is emphasized.