Traditionally refraction still serves as the basis for spectacle prescriptions. This is the result of a two-part process:
- Objective refraction: The refractive errors of the eye are first estimated objectively using retinoscopy or an automated refractor followed by
- Subjective refraction: A prescription is subjectively refined by an optometrist, allowing the patient to compare their vision through trial lenses, using either a refractor head or trial frame.
Subjective refraction uses different spectacle lens powers to correct refractive errors (such as myopia, hyperopia or astigmatism) in the eye, which are called low order aberrations, LOAs.
However, due to variations in pupil size with varying light conditions, other types of aberrations appear, affecting the quality of vision. These affect the final quality of vision experienced by the wearer. This is referred to as high order aberrations, HOAs (e.g. coma, trefoil, and spherical aberration).