Kozhaya et al. are to be congratulated on their study clarifying the impact of spherical aberration (SA) on visual acuity and depth of focus in pseudophakia.1 We have provided a figure that graphically illustrates their findings and demonstrates why there was no “expansion of the depth of field” with positive SA and why “they did not see a near benefit” (Figure 1).

Figure 1.:

Panel A is for the target at infinity (distance), panel B for intermediate (67 cm), and panel C for near (40 cm). In each panel, the top lens system has positive ocular SA, the middle zero, and the bottom negative ocular SA. The top and bottom lens systems have equal magnitudes of longitudinal ocular SA (0.35 μm) with their “best foci” shown as vertical green lines. The black double arrows represent the 5 ocular SAs tested for each distance. As the target moves to intermediate and near distances, the negative ocular SA lens system provides the clearest image. SA = spherical aberration

In our figure, there are 3 panels (A, B, and C), each of which have 3 lens systems: positive ocular SA at the top, zero ocular SA in the middle, and negative ocular SA at the bottom. Each lens system has a normal cornea with an average positive SA of + 0.28 μm and intraocular lenses (IOLs) of varying SA, to produce the positive (+0.35 μm), zero, and negative SA (−0.35 μm).2

In panel A, the target is placed at optical infinity and forms a paraxial image along the vertical yellow dashed line at 0 diopters (D) for all 3 lens systems. Five vertical blue dashed lines used in their study are shown which correspond to +0.4 μm, +0.2 μm, 0.0 μm, −0.2 μm, and −0.4 μm labeled at the top of the panel and the corresponding defocus in diopters at the bottom (+2.0 D, +1.0 D, 0.0 D, −1.0 D, and −2.0 D) for a 6 mm aperture (pupil). The marginal rays (3 mm from optical axis) cross at the marginal focus. The longitudinal ocular SA is the distance from the marginal focus to the paraxial focus for each of the 3 lens systems. The “best focus” for each of the 3 lens systems from the standpoint of wavefront aberration is at the midpoint of the longitudinal SA and is shown as the green vertical lines for each lens system.3,4

In the Kozhaya study, the lens system of the cornea and monofocal IOL had +0.35 μm of positive ocular SA, and the other 3 had zero ocular SA.5,6 For the 3 distances: infinity (panel A), intermediate (panel B), and near (panel C), 5 targets were presented with the 5 ocular SAs corresponding to their respective best foci of +1.0 D, +0.5 D, 0.0 D, −0.5 D, and −1.0 D shown as black double arrow images at the bottom (I1 to I5). In panel A, I1 is 0.025 μm to the left of the best focus of the positive ocular SA (+0.175 μm) lens system, I3 is exactly in focus for zero ocular SA, and I5 is 0.025 μm to the right of best focus for negative ocular SA lens system (−0.175 μm). The best focus for the zero ocular SA lens system is a point (at 0.0 D) because the marginal and paraxial foci are coincident, but the positive and negative ocular SA lens systems at their best foci are blurred circles. The zero SA would therefore have the clearest image for a target at distance and clinically has been shown to be approximately 1 logMAR visual acuity line better than an ocular SA of ±0.3 μm for a normal mesopic pupil (∼4.5 mm).1,5

In panel B, when the target is moved near the eye to the intermediate distance (67 cm, orange vertical line), the image moves to the right. This can also be accomplished with a −1.50 D lens in front of the eye with the target at infinity. We see that the image of the intermediate target is farthest from the positive SA's best focus and is closest to the negative SA's best focus. In panel C, as the target is moved to near (40 cm or −2.50 D lens in front of the eye), red vertical line, it is still closest to the negative SA best focus and even farther from the positive SA best focus.

For emmetropia, negative ocular SA will have the clearest image at intermediate and near distances but sacrifice 1 line of best corrected distance acuity because of the enlarged blur circle.1,5 There is no benefit at intermediate or near for positive ocular SA, and there is the same reduction in distance acuity as with negative ocular SA. An additional important factor that favors negative ocular SA when measuring near vision performance is the pupil miosis from the synkinetic reflex. The constriction of the pupil isolates to the central portion of the IOL, which is stronger for negative, weaker for positive, and neutral for zero ocular SA.7

Conversely, positive SA can extend depth of focus in a hyperopic direction because the best focus with positive SA is in front of the retina. This can be exploited by targeting the eye for myopia. However, because there is no synkinesis reflex, this approach lacks the advantage of pupil miosis that occurs with negative SA.

1. Kozhaya K, Kenny P, Esfandiari S, Wang L, Weikert M, Koch D. Impact of spherical aberration on visual quality and depth of focus in pseudophakic eyes. J Cataract Refract Surg 2023;50:24–29 2. Wang L, Dai E, Koch DD, Nathoo A. Optical aberrations of the human anterior cornea. J Cataract Refract Surg 2003;29:1514–1521 3. TelescopeѲptics.net. 4. Intrinsic telescope aberrations. 4.1. Spherical aberration. Available at: https://www.telescope-optics.net/spherical1.htm#specified/. Accessed December 23, 2023 4. Smith WJ. Modern Optical Design: The Design of Optical Systems. New York, NY: McGraw-Hill Book Company; 1966:295 5. Holladay JT, Piers PA, Koranyi G, van der Mooren M, Norrby NE. A New intraocular lens design to reduce spherical aberration of pseudophakic eyes. J Refract Surg 2002;18:683–691 6. Barbero S, Marcos S, Jimenez-Alfaro I. Optical aberrations of intraocular lenses measured in vivo and in vitro. J Opt Soc Am 2003;20:1841–1851 7. Zheleznyak L, Jung H, Yoon G. Impact of pupil transmission apodization on presbyopic through-focus visual performance with spherical aberration. Invest Ophthalmol Vis Sci 2014;55:70–77