They retain their ability to see at distance without correction with age. Astigmatism Astigmatism is the term used to describe uneven refractive power over different parts of the cornea. Essentially, the cornea is warped. Night vision is blurred as the pupil dilates and light passes through the warped periphery of the cornea. The degree of warping is measured in diopters versus a 360 degree axis with 0 degrees at the the top of the cornea.
Corrective lenses have a reciprocal warping to compensate.
Vision Correction Prescriptions Prescriptions for corrective lenses may have up to four components. The first component is the refractive error. This measures the amount of curve in the corrective lenses to compensate for myopia or hyperopia. Since the myopic cornea has excessive refractive power or diopters, the prescription is written in terms of negative diopters of correction, e.g., -2.25. The second and third components of a refractive prescription deal with astigmatism and are called cylindrical errors.