Refractive errors are categorized as Spherical errors and Cylindrical errors:
- Spherical errors occur when the optical power of the eye is either too large or too small to focus light on the retina. People with refractive error frequently have blurry vision.
• Nearsightedness: The optics is too powerful to the length of the eyeball for one has myopia or nearsightedness. This can arise from a cornea or crystalline lens with too much curvature (refractive myopia) or an eyeball that is too long (axial myopia). Myopia can be corrected with a concave lens, which causes the divergence of light rays before they reach the cornea.
• Farsightedness: The optics is too weak to the length of the eyeball for one has hyperopia or farsightedness. This can arise from a cornea or crystalline lens with not enough curvature (refractive hyperopia) or an eyeball that is too short (axial hyperopia). Hyperopia can be corrected with convex lenses, which causes the light rays to converge prior to hitting the cornea.
• Presbyopia: The flexibility of the lens declines, typically due to age. The individual would experience difficulty in near vision termed as presbyopia. It can often relieved by reading glasses, bifocal, or progressive lenses.
- Cylindrical errors occur when the optical power of the eye is too powerful or too weak across one meridian, such as the corneal curvature tends towards a cylindrical shape. The angle between that meridian and the horizontal is known as the axis of the cylinder.
• Astigmatism: A person with astigmatic refractive error perceives lines of a particular orientation less clearly than lines at right angles to them. This defect can be corrected by refracting light more in one meridian than the other. Cylindrical lenses serve this purpose.