Lower-order aberrations (LOAs) such as myopia (positive defocus), hyperopia (negative defocus) and astigmatism are well known to optometrists, but more subtle and complex refractive errors, known as higher-order aberrations (HOAs), require a different approach to diagnosis and correction.

HOAs are changes to the waves of light as they pass through the optical structures of the eye (tear film, cornea, aqueous, lens and vitreous humour). All eyes have some form of HOA but, in some cases, they result in clinically significant changes, resulting in a less than perfect image being perceived. The aberrations can affect a person’s contrast sensitivity and sharpness and cause changes in magnification and focus. Examples include coma, spherical aberration and trefoil. Symptoms of these aberrations can include blurred vision, difficulty seeing at night, double vision, ghost images and halos.