Understanding spherical aberration

Optical wavefront aberrations are imperfections in the way a lens or optical system focuses light. They occur when incoming light rays fail to meet at a single focal point, producing a blurred or distorted image instead of a perfectly sharp one. This mismatch means that light cannot converge into a single clear image, slightly blurring the retinal image.

Spherical aberration (SA) is a type of higher-order aberration — a more complex distortion of the wavefront of light, that goes beyond simple defocus or astigmatism. It occurs when light rays passing through the outer edges of the cornea or lens do not focus at the same point as those passing through the centre.

SA is expressed mathematically using Zernike polynomials, which describes the shape of the overall wavefront and breaks it down into individual components. 

On a Zernike map, SA is represented as a shape resembling an inverted bowl, and illustrates how light rays are focused differently across the pupil. The value assigned to this shape — positive or negative — indicates whether peripheral light rays focus in front of or behind the retina. 

A positive value means the outer rays focus more anteriorly, while a negative value means the outer rays focus more posteriorly.

In an optically perfect system, all incoming rays would converge at a single focal point. In reality however, spherical aberration exists, where rays that enter parallel away from the optical axis (paraxial rays) are refracted by a different amount than those passing through the centre.

In an eye which has positive SA overall, peripheral rays (outer paraxial rays) are refracted more strongly than central rays, bringing them to focus slightly in front of the retina. If it were mostly negative SA, peripheral rays are refracted less strongly, focusing behind the retina instead.

The result in both cases is a longitudinal spread of focus, where the image is formed over a range of depths, rather than a single plane (if it were an optically perfect system). This reduces the overall quality of the retinal image but increases the eye’s depth of focus – extending the range of distances that the image can remain relatively sharp.

In normal human eyes, spherical aberration is the most prominent higher-order aberration. Average values for a human eye are approximately +0.08 ± 0.07 µm for a 5 mm pupil1 and +0.10 ± 0.10 µm for a 6 mm pupil.2-4

Most of the eye’s inherent SA originates from the cornea, which contributes about +0.12 µm at a 5 mm pupil and +0.28 µm at a 6 mm pupil.5 This positive corneal aberration is partly offset by the crystalline lens, which adds a small amount of negative SA, at around -0.01 µm for a 5 mm pupil.1

During accommodation, the lens contributes more negative SA to the system. This equates to roughly –0.04 µm per dioptre of accommodation for a 5 mm pupil, shifting the overall system from positive to negative SA as the eye increases its accommodation.6

As with all biological measurements, there is significant variation between individuals, and the values above represent population averages reported across multiple studies. However, myopic children3 and young adults have been shown to have lower and more variable SA compared to emmetropes.7,8

Animal research has demonstrated a strong link between optical defocus and eye growth. When images are focused behind the retina (hyperopic defocus), this stimulates eye elongation and subsequently myopia progression. 

In contrast, when focus occurs in front of the retina (myopic defocus), eye growth slows. This relationship underpins the principle of peripheral myopic defocus, where the peripheral retina experiences myopic defocus while the macula remains in clear focus. 

Orthokeratology (OK) and centre-distance soft multifocal contact lenses (MFCL) are both effective interventions for controlling myopia, largely because they induce peripheral myopic defocus. 

However, both OK and centre-distance MFCLs have optical profiles that also increase positive SA. Because the peripheral refractive power is greater than the central refractive power, peripheral rays are brought to focus more anteriorly than central rays. This increases the longitudinal spread of focus, and if measured by a wavefront sensor, would show an increased amount of positive SA.

A proposed mechanism is that inducing higher positive SA may reduce the impact of accommodative lag, which in turn could contribute to myopia progression. 

In the human eye, a small lag of accommodation is common during near tasks. This means the focal point of the visual system falls slightly behind the retina, effectively creating mild hyperopic defocus. There is debate however, about whether accommodative lag may be a cause or a feature of myopia.

Orthokeratology and centre-distance multifocal contact lenses have been shown to effectively slow myopia progression, and both increase positive SA as part of their optical profile. However, this association does not necessarily mean that SA itself is responsible for slowing eye growth. 

Current evidence suggests that these treatments are effective primarily because they create peripheral myopic defocus, a well-established signal for slowing myopia progression. The accompanying increase in SA (due to peripheral defocus) could either contribute to the myopia control effect, or could simply be a coincidence.

Several studies have attempted to harness increased positive spherical aberration as a mechanism for myopia control using multifocal contact lens designs, with mixed results. Ongoing research continues to investigate this relationship, making SA an important concept to understand in the context of myopia management.

• Spherical aberration occurs when central and peripheral light rays focus at different depths, creating a longitudinal spread of focus.
• Increasing spherical aberration extends the depth of focus, but at the cost of reduced retinal image quality.
• Orthokeratology and soft multifocal contact lenses increase positive spherical aberration as part of their geometry, but it remains inconclusive whether positive SA itself protects against myopia progression.