Impact of Uncorrected Astigmatism on Image Quality With Dual-focus Optics—A Modelling Study

Abstract Purpose Compare the impact of uncorrected astigmatism in a single-vision (SV) and a dual-focus (DF) optical lens models at the centre and limits of the interval of Sturm (IOS). Methods With the rule (WTR) astigmatism (up to 2.0 D) was added to SV and DF models. Wavefronts were constructed from Zernike polynomials over the full pupil (SV) or on a zone-by-zone basis (DF). The DF model included an annular zone with +2.0 D of added power and a typical decentration from the pupil centre. Geometrical ray tracing and wave optics were used to compute point-spread functions (PSF), modulation transfer functions (MTF) and simulated letter images at the centre, anterior and posterior limits of the IOS. Results Simple line foci were observed with the SV model, but more complicated PSFs containing circular, elliptical and double orthogonal line components were created when astigmatism was added to the DF model. Simulated images in both the SV and DF models were degraded with increasing levels of astigmatism. In the absence of astigmatism, lens-only MTFs were reduced by ~45% in the DF model. As astigmatism levels increased, the MTF values were reduced in both models, reducing the differences between them. In the presence of ocular aberrations, MTFs with astigmatism >1.0 D were almost identical in the SV and DF models. Conclusions The modelling shows that uncorrected astigmatism remains a major contributor to image degradation in eyes corrected with a DF contact lens, suggesting that astigmatic eyes being fitted with dual-focus or bifocal contact lenses would benefit from a toric correction.