Positive or negative spherical aberration- Which of the two provides better depth of focus by EDOF IOLs?
- Jun 4, 2025
- 8 min read
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 or negative 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 therefore is not a fixed point on the image plane, and shift according to the pupillary diameter and the location of the object, either at infinity or any distance closer to the eye.
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:
In an negative EDOF IOL, the best focus is before the paraxial focal point. In a positive EDOF IOL, like Rayner EMV, the best focus is after the paraxial focal point. Having understood this, the below picture will help you understand what happens in case of myopic or hyperopic outcome with positive or negative EDOF IOLs.
