Positive or negative spherical aberration- Which of the two provides better depth of focus?
- Subhabrata Bhattacharya
- Jun 4
- 6 min read
Updated: Jun 6
In the article in this blog 'Spherical Aberration, Q factor & choice of IOL' (https://www.quickguide.org/post/spherical-aberration-asphericity) I have provided an understanding on the subject of spherical aberration, the concept of positive and negative spherical aberration and how an IOL can be chosen for cataract patients based on the Q factor of the cornea. In this article, I will go a little deeper in understanding the 'best focus' or 'circle of least confusion' with spherical aberration and answer the following question:
Positive or negative spherical aberration, between the two what provides a better depth of focus?
To understand this we will first try to understand the concept of best focus with spherical aberration. In image 1, the axial or longitudinal spherical aberration is described as the distance between the paraxial focus and the marginal focus. We know spherical aberration provides depth of focus, but also degrades the image quality. This degradation of image quality is the result of the blur circle on the retina, as the marginal rays fall further away from the paraxial focal point. This is what typically happens in positive spherical aberration.
When the pupil is small, for example in day time, the marginal rays of light do not reach the retina. As a result the image is sharp with a significantly lower blur circle. As we enter into a mesopic condition, the pupil is large, as a result of which marginal rays of light that now pass the pupil fall in front of the paraxial focal point, creating a longitudinal positive spherical aberration.
The place where the patient best focus shift, that is the place where the image is a circle of least confusion is shown in image 2.
To find the best focus of the patient in a dilated pupil in presence of positive spherical aberration (SA), follow the steps:
A) Identify the point where the marginal ray is meeting the paraxial ray (marked by green arrow)
B) Join this point to the paraxial focal point (green line).
C) Now identify the point where the inferior marginal ray (red line) is meeting the caustic (green line)
D) The point where the two lines meet is the 'best focus' for the patient.
The best focus is the place where an object at infinity will have the least blur circle, and the image would therefore be acceptable to the patient.
The best focus could be connected to the circle of least confusion concept from astigmatism. This is the place where the image of an object at infinity will have the least blur and therefore fairly acceptable to the patient. Thus with increasing amounts of ocular positive spherical aberration, the best focus shifts further away from the paraxial focal point. The more the spherical aberration, the larger the blur circle of the best focus and larger the drop in image quality. Night myopia is a term associated with patients who experience blurred vision in low light condition even though in the day time, they are normal. The shift in best focus of the patient as the pupil dilates explains such condition experienced by emmetropes with large spherical aberration.
Negative spherical aberration is a condition wherein the marginal rays of light fall beyond the paraxial rays of light (Image 3 top). Thus the best focus of the patient will be beyond the paraxial focal point (Image 3 bottom). Thus the following conditions will happen:
For a emmetrope, with negative spherical aberration, the best focus will be on the hyperopic side.
For an emmetrope with positive spherical aberration, the best focus will on the myopic side.
Karolinne Maia Rocha and co-authors found that higher positive spherical aberration provided better DCNVA than negative spherical aberration(1). In an earlier study by the same first author in 2007, SN60AT ( a non aspheric spherical IOL) was shown to provide better DCNVA over the Acrysof IQ aspheric lens. Note, a spherical IOL adds to the positive spherical power for a cornea which is generally prolate. However, using adaptive optics (simulate different types of vision, including how aberrations affect vision), Rocha and co-authors in 2009 showed that both positive and negative spherical aberration could result in depth of focus for the patient(2). As described above, for an emmetrope with negative SA, the best focus will shift to the hyperopic side ( and the opposite for positive SA) the authors found that the shift of the best focus (authors describe as 'center of focus') is in the direction of the sign of the induced spherical aberration, that is best focus shifts on the positive side with positive SA and to the hyperopic side with negative SA. This may explain why intermediate or near vision improvement is noticed with positive SA rather than negative SA.
Contrary to work of Rocha and co-authors with Adaptive Optics, Khozaya and co-authors found no benefit of inducing positive SA in increasing depth of focus. The authors noted that only negative SA induced depth of focus. Positive SA decreased CDVA for both the enhanced monofocal and continuous range-of-vision IOLs and DCNVA for the monofocal IOLs with zero SA(3). Damiel Gatinel (4) explains the surprise finding of Khozaya and co-authors. Gatinel and Stern explains that inducing positive Zernike SA with adaptive optics introduces a hyperopic defocus in the center of the pupil, whereas inducing negative Zernike SA results in a myopic shift. As the paracentral rays of light shifts towards the myopic side with induction of negative SA, the patient's depth of focus improves, giving an impression that only negative SA works. Gatinel and Stern advice that if adaptive optics have to be used to simulate negative and positive SA, then the patient's far focus has to be corrected for distance to negate any shift in focus of paraxial rays of light. Only then can we understand the effect of negative or positive SA on depth of focus.
Juan Tabernero and co-authors(5), used adaptive optics to determine what amount of negative spherical aberration would help in near and intermediate without significantly affecting the distance. Starting from 0.07 to 0.3 mm of spherical aberration, the increase in depth of focus followed a linear trend, with a rate of change of 0.4 D of depth of focus per 0.1 mm of spherical aberration. The authors showed that on average, when ocular spherical aberration increases more than 0.15 mm (all spherical aberration data in this section refer to a pupil diameter of 4.5 mm), the far distance visual acuity is worse than 0.15 logMAR units, which is hardly clinically acceptable. To be able to achieve a far visual acuity better than this value, the most effective EDOF IOL (on average) to extend the DoF would correspond to values of spherical aberration that range between 0.07 mm and 0.15 mm(5). Below is a video description from their study.
In this article I have tried to throw some light on negative and positive SA, and what may be more useful in providing a depth of focus. More importantly, what amount of SA is tolerable to patient for far, and yet provide some depth of focus for the intermediate and near.
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References:
Karolinne Maia Rocha, MD, PhD, Larissa Gouvea, MD, George Oral Waring, IV, MD,
Jorge Haddad, MD, Static and Dynamic Factors Associated with Extended Depth of Focus in Monofocal Intraocular Lenses, https://doi.org/10.1016/j.ajo.2020.04.014
Karolinne Maia Rocha 1, Laurent Vabre, Nicolas Chateau, Ronald R Krueger, Expanding depth of focus by modifying higher-order aberrations induced by an adaptive optics visual simulator, DOI: 10.1016/j.jcrs.2009.05.059
Kozhaya, Karim MD; Kenny, Peter I. BS; Esfandiari, Saina OD; Wang, Li MD, PhD; Weikert, Mitchell P. MD; Koch, Douglas D. MD, Effect of spherical aberration on visual acuity and depth of focus in pseudophakic eyes, Journal of Cataract & Refractive Surgery 50(1):p 24-29, January 2024. | DOI: 10.1097/j.jcrs.0000000000001314
Comment on: Impact of spherical aberration on visual quality and depth of focus, https://doi.org/10.1097/j.jcrs.0000000000001551
Juan Tabernero, PhD, Carles Otero, PhD, John Kidd, BSc, Laura Zahiño, OD, Ana Nolla, OD, Jose Luis Güell, MD, Pablo Artal, PhD, Shahina Pardhan, PhD, Depth of focus as a function of spherical aberration using adaptive optics in pseudophakic participants, J Cataract Refract Surg 2025; 51:307–313
