Reply: Impact of spherical aberration on visual quality and depth of focus

We are pleased to respond to Stern and Gatinel's comments on our recent article.1 The illustration was intended to graphically demonstrate the findings in the article by Kozhaya et al.2 Monocular adaptive optics visual simulation was performed with the VAO (Voptica S.L.) with the pupil fixed at 6 mm. Ocular spherical aberrations (SAs) and refractions were measured at baseline ocular SA, with no further adjustments after SA induction. This approach effectively aligns with the paraxial focal plane and corresponds to the positive Seidel SA in the diagram provided by Stern and Gatinel. Our original illustration precisely corresponds to the findings by Kozhaya et al., reported in Table 2, and explains why the authors only found a benefit with zero ocular SA (as seen with intraocular lenses [IOLs] with negative SA) and no improvement with positive ocular SA (as in IOLs with positive SA). We agree that registering the optical systems at their best focus more accurately reflects clinical practice and is the most appropriate method for comparing the optical performance of different ocular systems. It is important to note that the optical systems reflect the ocular SA and include both the corneal and lenticular aberrations. The mean ocular SA in monofocal lenses with zero SA was 0.34 μm. With TECNIS aspheric IOLs, the ocular SA was near zero.2 The study did not include any systems with negative ocular SA. For zero ocular SA, the best focus is a single point, whereas, for positive or negative ocular SA, the focus is spread over the longitudinal SA. Laboratory and clinical studies have shown that zero ocular SA results in a one-line improvement in corrected distance visual acuity and a 30% to 40% enhancement in contrast sensitivity function.3 However, the longitudinal SA spread at the best focus for positive and negative ocular SA provides both SAs with myopic rays, aiding in near vision and increasing the depth of field. The more significant factor relates to the pupil miosis from the synkinetic reflex when viewing a near object. In the standard refracting lane, the brightness is nominally 85 cd/m2 and the mean pupil diameter in a 72-year-old patient is 4.5 mm.4 When viewing a near object at <66 cm, the pupil constricts to ∼2.25 mm. Reducing the pupil diameter by half increases the depth of field by a factor of 2. This represents a 3-line improvement in visual acuity. Pupil miosis with near vision is the most significant factor in improving near vision, and it equally affects positive, zero, and negative SA. However, a difference lies in the central power of the ocular systems with pupil constriction. As illustrated in Figure 1, the central portion of positive SA is hyperopic (panel B); for zero ocular SA, it is emmetropic (panel A); and for negative ocular SA, it is myopic (panel C). The hyperopic change is detrimental and reduces the central power by approximately 0.50 to 0.75 diopters. Similar changes have been reported after myopic LASIK, where the induction of positive SA often leads to a reduction in near vision for presbyopic patients.5Figure 1.: Panel A is for zero ocular SA, panel B is for positive ocular SA, and panel C is for negative SA. The lower two lens systems have equal magnitudes of longitudinal ocular SA (0.34 μm) at their best foci with equal and opposite amounts of defocus. With pupil miosis, positive ocular SA becomes hyperopic and negative ocular SA becomes myopic. The myopic shift provides an advantage of approximately 0.50 to 0.75 diopters for near and the hyperopic shift a disadvantage of the same magnitude. SA = spherical aberration