What ratio between distance and near power area do we need to provide an acceptable quality of distance and near vision with bifocal contact lenses?

Abstract

 

Purpose

 

It is assumed that the ratio of distance to near power area given in a bifocal lens should be 50/50. The purpose of this study was to determine the impact of this ratio on the subjective quality of vision.

 

Methods

 

The subjective image quality was assessed using a continuous 5-items grading scale. Fourteen young normally sighted subjects judged three times the quality of computationally blurred images (three high-contrast 20/50 letters) through a 3 mm artificial pupil limiting the impact of their aberrations. The simulated images were calculated for a 4.5 mm pupil diameter from -4 to +2 D each 0.25 D and with 10 various center-near bifocal contact lenses.The addition of the bifocal contact lenses was 2.5 D. The varying parameter between the 10 optical profiles was the area covered by the addition ranging from 0% (i.e. single distance correction) to 90% (i.e. a 2.1 mm center-near zone diameter) with a step of 10%. The images were judged in a random order.

 

Results

 

To quantify the ability of bifocal lenses to extend the depth-of-focus (i.e. range of proximities over which an acceptable level of vision is obtained), we calculated the area under the through-focus subjective quality of vision curve higher than 2 (i.e. level from which the quality of vision becomes acceptable). This criterion was normalized by the naked eye condition. A ratio higher than 1 represents a benefit of wearing the bifocal lens. The benefit averaged on the 10 profiles is highly subject dependent (i.e. from 0.8 to 1.9) explaining why some subjects could not be satisfied with such corrections. Averaged on the subjects, the best benefit (i.e. 1.42) was obtained with the 40% bifocal lens (i.e. 40% of the pupil area is covered by the addition). The 20%, 30% and 50% profil gave quite comparable benefit (i.e. around 1.35). The inter-individual standard deviation is highly negatively dependent (r²= 0.97) on the pupil area covered by the addition.

 

Conclusions

 

The 50/50 criterion doesn’t seem so crucial, other ratios providing similar or better benefits. Through-focus image quality scores measured on simulated images could be useful to predict the acceptability of bifocal lenses and the level of satisfaction of a given subject.

 

 

Benefit as a function of pupil's area covered by the near power

 

 

Interindividual SD of the benefit as a function of pupil's area covered by the near power

 

Keywords: 653 presbyopia • 477 contact lens