Astigmatism is an asymmetrical refractive error. In an eye with perfect vision, the cornea curves evenly like a soccer ball, focusing inbound rays onto a single spot on the retina. With astigmatism, the cornea resembles an oblong football, featuring steeper and flatter meridians. Light entering horizontally bends at a different angle than light entering vertically.
During daylight hours, this imperfection remains manageable. Bright ambient light forces your pupil to contract to roughly 2 to 3 millimeters, functioning like a narrow camera aperture that blocks peripheral distortions. Night changes the optical geometry. Under dim skies, natural pupil dilation expands the aperture to 6 to 8 millimeters, exposing the warped outer perimeters of your corneal curvature.
This wide-aperture state triggers the starburst effect and the distinctive halo effect around lights. Point sources of luminance stretch into erratic flares, spiked rays, and smeared lines across your visual field. Instead of registering a distinct set of brake lights 100 feet ahead, your brain must decode an indistinct wash of red photons, cutting low-light visual acuity and delaying emergency braking reactions.
