Anomalies of refraction
An eye with no vision defects is said to be emmetropic. Light rays coming from a faraway object at infinity focus directly on the retina, producing a clear, sharp image.
When the image does not focus on the retina, it is blurry. This is the result of an error of refraction, or an ametropia.
In the myopic eye, light rays from a faraway object at infinity focus in front of the retina. A distant object is perceived as being blurry.
In the hypermetropic eye, light rays from a faraway object at infinity focus beyond the retina. The faraway object is perceived as blurry, and the closer it comes, the blurrier it gets.
In the astigmatic eye, light rays focus at different points in front or beyond the retina, resulting in a distortion of the image at any distance. An object is blurry regardless of distance.
In the presbyopic eye, vision focus for reading or performing a task close to the eyes becomes increasingly difficult. It is hard for a person with presbyopia to see objects at short distances because the lens of the eye has lost its elasticity, and no longer accommodates differences in distance. Most people over the age of 40 have some symptoms of presbyopia.
Eyeglasses, contact lenses, or refractive surgery correct or reduce refractive errors, by changing the focus of the rays of light so that it falls on the retina, or closer to the retina.