Myopia, hyperopia, and regular astigmatism are all lower-order (second-order) aberrations that can be expressed as wavefront aberrations. Myopia produces positive defocus (Fig 1-2), whereas hyperopia produces negative defocus. Regular (cylindrical) astigmatism produces a wavefront aberration that has orthogonal (ie, facing at right angles) and oblique components (Fig 1-3). Other lower-order aberrations are non–visually significant aberrations known as first-order aberrations, such as vertical and horizontal prisms and zero-order aberrations (piston).
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.