Myopia, hyperopia, and regular astigmatism are all lower-­order, or second-­order, aber­ rations. Myopia produces positive defocus (see Figure 1-4), whereas hyperopia produces negative defocus. Regular (cylindrical) astigmatism produces a wavefront aberration that has orthogonal (ie, facing at right ­angles) and oblique components (see Figure 1-5). Other lower-­order aberrations are non–­visually significant aberrations known as first-­ order aberrations, such as vertical and horizontal prisms and zero-­order aberrations.

Wavefront aberration is strongly affected by pupil size, with higher-­order aberrations be­ coming more apparent as the pupil dilates. Higher-­order aberrations also increase with age, although the clinical effect is thought to be balanced by the increasing miosis of the pupil with age. Although lower-­order aberrations decrease ­after ­laser vision correction, higher-­order aberrations, particularly ­spherical aberration and coma, may increase ­after conventional surface ablation, ­laser in situ keratomileusis (LASIK), or radial keratot­ omy (RK) for myopia. (See the section “Effect of excimer ­laser ablation on higher-­order aberrations.”)

When peripheral light rays passing through a lens or the cornea focus in front of more central rays, the effect is called ­spherical aberration (see Figure 1-6 in this volume and also BCSC Section  3, Clinical Optics). Clinically, this radially symmetric fourth-­order aberration is the cause of night myopia and is commonly increased ­after RK and myopic LASIK and surface ablation. It results in halos around point images. ­Spherical aberration is the most significant higher-­order aberration. Although it may increase depth of field, it decreases contrast sensitivity.

With coma, a third-­order aberration, light rays at one edge of the pupil come into focus before ­those at the opposite edge do. The rays entering the system do not focus on a plane; rather, one edge of the incoming beam focuses ­either in front of or ­behind the opposite edge of the beam (see Figure 1-7). The image generated by an incoming light beam pass­ ing through an optical system with a coma aberration would appear “smeared,” looking somewhat like a comet, with a zone of sharp focus at one edge tailing off to a fuzzy focus at the opposite edge of the beam. Coma is common in patients with decentered corneal grafts, keratoconus, and decentered ­laser ablations.

Trefoil, also a third-­order aberration, can occur ­after refractive surgery. Trefoil pro­ duces less degradation in image quality than does coma of similar RMS magnitude (see Figure 1-8).