Why Do I See Starbursts and Halos Around Lights at Night? Astigmatism vs Pupil Dilation

The number one reason for starburst headlights is astigmatism. A normal cornea is spherical like a basketball. In astigmatism, the cornea is shaped like an American football, having two different curvatures at right angles to each other:

$$F_{\text{steep}} = \frac{n-1}{r_{\text{vertical}}} \quad \ne \quad F_{\text{flat}} = \frac{n-1}{r_{\text{horizontal}}} \implies \mathbf{\text{Light focuses into two separate focal lines (Sturm's Conoid)}}$$

Instead of focusing a point source of light into a pinpoint dot on your fovea, the light is stretched into an elongated streak. Even a tiny, uncorrected cylinder error of just $-0.50\ \text{D}$ or $-0.75\ \text{D}$ causes headlights to smear into massive, spiky starbursts.

During daylight, your pupil is small ($2.0\text{ to } 3.0\ \text{mm}$), acting like a pinhole camera that blocks marginal light rays. But at night, your pupil opens to $6.0\text{ to } 7.0\ \text{mm}$:

$$W_{040} \propto r_{\text{pupil}}^4 \implies \text{When pupil doubles from } 3\text{mm to } 6\text{mm}, \ \mathbf{\text{Spherical Aberration increases by } 16 \times!}$$

Light passing through the peripheral edge of your natural cornea bends more sharply than central light, creating a hazy ring or halo around point light sources.

Getting crisp night vision requires two steps:

1. Exact Cylinder & Axis Correction: A comprehensive refraction determines your exact astigmatism down to the single degree of axis ($1^\circ$), collapsing the Sturm's interval back to a single sharp focal point.
2. Aspheric Lens Geometry: Lenses surfaced with aspheric or freeform front curves flatten toward the edge, directly canceling out ocular spherical aberration ($Z_4^0$) when your pupils dilate at night.