is said to have ametropia or be ametropic. This eye cannot focus parallel rays of light (light from distant objects) on the retina, or needs accommodation to do so. The word "ametropia" can be used interchangeably with "refractive error". Types of ametropia include myopia, hyperopia and astigmatism. They are frequently 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: When the optics are too powerful for the length of the eyeball 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: When the optics are too weak for the length of the eyeball, 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). This can be corrected with convex lenses, which cause light rays to converge prior to hitting the cornea. Presbyopia: When the flexibility of the lens declines, typically due to age. The individual would experience difficulty in near vision, often relieved by reading glasses, bifocal, or progressive lenses. Cylindrical errors cause astigmatism, when the optical power of the eye is too powerful or too weak across one meridian, such as if 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 sees 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. Other terminology include anisometropia, when the two eyes have unequal refractive power, and aniseikonia which is when the magnification power between the eyes differ. Refractive error may be quantified as the error of a wavefront arising from a person’s far point, compared with a plane, or zero vergence, wavefront compared at an appropriate reference plane. The reference plane may be a real plane such as the spectacle plane or the corneal plane,