Preferred Image Focus Location for Best Visual Acuity and Astigmatic Blur Adaptation

Abstract

Purpose :
In an astigmatic optical system light does not focus at a single point. There are primary and secondary focal lines with a Circle of Least Confusion (CoLC) at the dioptric center. Due to the symmetrical nature of the conoid, it is sometimes assumed that visual acuities (VA) are similar equidistant from the CoLC and best VAs are centered at the CoLC. This experimental study tested the hypothesis that VAs are not equivalent in hyperopic and myopic astigmatism. We also tested whether blur adaptation would improve VAs for subjects with astigmatism.

Methods :
Subjects were students (n=63, 37 females, 26 males, age=23.9±2.2years old) at the Kentucky College of Optometry. Manifest subjective refraction was performed to measure refractive error and BCVA. If no astigmatism was present, axis was set to 180. After best correction was achieved, cylinder values were adjusted by +2.00 Diopters Cylinder (DC), +1.00DC, -1.00DC, and -2.00DC in the axis of the subjective manifest refraction. Visual acuities were measured at each step. Subjects were divided into an astigmatic group A (n=12,≥0.75DC) and a non-astigmatic group B (n=51,<0.75DC) for statistical analysis.

Results : The groups had similar LogMAR BCVA (Group B=-0.06 vs Group A=-0.04, P-value=0.53). Group A had less reduction in LogMAR visual acuity when presented with astigmatic defocus of -1DC, -2DC, +1DC, and +2DC by a mean value of 0.05 (P-Value=0.024). The data also showed that both groups had better acuities for hyperopic astigmatism than for myopic astigmatism (0.06 LogMAR vs 0.16 LogMAR for ±1DC, P-Value=7.2E-05; 0.24 LogMAR vs 0.37 LogMAR for ±2DC, P-Value=4.2E-06). A quadratic regression of the data indicated a negative astigmatic shift in both groups but greater for the astigmatic group.

Conclusions : The study confirmed that subjects with astigmatism (Group A) showed less loss of VA with astigmatic blur, indicating improved astigmatic blur interpretation. It also confirmed that VAs were different between hyperopic astigmatism and myopic astigmatism with the VA of hyperopic astigmatism being better. The shift in improved VAs in the quadratic regression towards hyperopic astigmatism might indicate that BCVA may not be located at zero astigmatism.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Graph showing Astigmatism in Diopters Cylinder (x-axis) vs LogMAR Visual Acuity (y-axis). The blue quadratic function trendline represents astigmatic subjects. The orange trendline represents non-astigmatic subjects.