Top: farsighted corrected using convex lens Bottom: nearsighted corrected using concave lens. An eye that has no<unk>  when viewing distant objects is said to have emmetropia or be emmetropic meaning the eye is in a state in which it can focus parallel rays of light (light from distant objects) on the retina, without using any accommodation. A distant object, in this case, is defined as an object located beyond 6 meters, or 20 feet, from the eye, since the light from those objects arrives as essentially parallel rays when considering the limitations of human perception. An eye that has<unk>  when viewing distant objects 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<unk> ". Types of ametropia include myopia, hyperopia and astigmatism. They are frequently categorized as spherical<unk> s and cylindrical<unk> s: * Spherical<unk> s occur when the optical power of the eye is either too large or too small to focus light on the retina. People with<unk>  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<unk>  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<unk>  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<unk> s 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<unk>  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<unk>  power, and aniseikonia which is when the magnification power between the eyes differ.<unk>  may be quantified as the<unk>  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, or an imaginary plane such as the first principal plane or the entrance pupil plane. In diopters, spherical<unk> s can be expressed as K=1/k, where k is the distance in meters from the reference plane to an eye’s far point, and K is the<unk>  in diopters. Thus, a person with myopia would have a negative<unk> , a person with emmetropia would have zero<unk>  and a person with hyperopia would have a positive<unk> . In the case of regular astigmatism,<unk>  needs to be expressed as 3 values: classically as sphere, cylinder and axis. However, it can also be expressed in vector terms, for example, M (mean sphere), J0 (With the rule/Against the rule astigmatism), J45 (oblique astigmatism).<unk> s containing higher order aberrations (sometimes referred to as irregular astigmatism) can be expressed for a given pupil size using wavefront<unk> s or optical path differences, often as coefficients for Zernike polynomials. A more subjective quantity visual acuity (expressed as a fraction) may be used, but there is no direct or exact conversion between the two.
Refractive error
Instrument myopia
Cataract
Ectopia 
Corneal ectatic disorders
Congenital cataract
Auditory fatigue
Ocular hypertension