The eye is a small ball but it is not a perfectly regular ball. Therefore there are small refractive errors that can be corrected with glasses.

Eyes without any refractive errors are rare. An eye without any refractive error sees clearly at far distance when its lens is relaxed. When such an eye looks at a close distance the lens has to accommodate, become thicker. If one then wants to look at a far distance the picture is first blurred for a brief moment and then becomes clear because it takes time before the lens is relaxed, flat again.

A. A hyperopic, far sighted, eye with its lens relaxed, the image is not in focus at the level of the retina (the lines do not cross at the retina).
B. The image becomes clear when the lens accommodates, becomes thicker and makes the light rays bend more so they now cross at the retina.
C. In a myopic, near sighted, eye the image of a distant object is in the vitreous and the image at the retinal level is blurred. Accommodation does not make the image clear. The myopic eye sees objects close by clearly.

Hyperopia is very common. The lens of a hyperopic eye is accommodated some when the eye looks at far distance and it has to be accommodated more when the eye looks at near. If the hyperopic eyes get tired when looking at reading distance the use of reading glasses can be started at any age. Reading glasses are plus lenses, thicker in the middle than at the edge.

Myopia cannot be compensated by accommodation. A myopic person sees better at distance when squeezing the eye lids to a narrow opening which functions as an optic slit. It is not dangerous. Myopia is corrected with minus lenses, thicker at the edges than in the middle of the lens.

Astigmatism means that lines in different directions are not in focus at the same time, e.g. the vertical lines are in focus at the retina but the horizontal lines are not, and when the horizontal lines are in focus the vertical lines are not. The image is made clear by using cylinder lenses.

Irregular refractive errors are common among the visually impaired. If the cornea has irregular surface or the lens focuses the light irregularly, parts of the image are clear and some other parts distorted or blurred. Sometimes contact lenses make the image much better but not always. In other cases corneal transplantation or removal of the cataractous lens improves the image quality.