Refractive errors arise as a result of anatomical variations within the eye that prevent the image being perfectly focused on to the retina. Such variations can occur within the cornea and the lens. Refraction is also affected by the depth of the anterior chamber of the eye and the length of the globe from front to back (axial length).

Where these errors arise, lenses have traditionally been used to modify the refraction of rays of light entering the eye. Increasingly, surgical techniques are being developed in order to carry out this function and so eradicate or minimise the need for extraocular corrective lenses.