To enable a sharp vision, light rays from an object placed in the visual field must converge to a single point of retina. The eye works like a camera – it has an opening in the front part (pupil), a focus mechanism (cornea and eye lens) and a light-sensitive portion in the back part (retina). Eyes affected by significant refractive errors such as myopia, hyperopia and astigmatism are not capable of focusing light in the intended retinal region.
In hyperopia, light rays are focused beyond retina (either because the eye has a short length or due to its lack of light convergence power), which makes both near and distant objects look blurred. Low hyperopia dioptres (grade or graduation) are common during childhood and typically there is no need to adjust it with the use of eyeglasses. Children can compensate it by themselves when using the natural focus mechanism (accommodation). Differently, higher dioptres may have consequences for development (visual acuity reduction, strabismus and amblyopia) and demand adjustment with convergent or positive (+) power eyeglasses, especially when associated with accommodative esotropia (a convergent strabismus form). Hyperopia may increase in early childhood and then decreases during preadolescence until the beginning of adolescence. Contact lenses and surgery are possible alternatives to eyeglasses at an appropriate age.
In myopia, light rays are focused before getting to retina either because the eye is very long or because it has an excessive convergence power. Distant objects look blurry and only become easier to see when getting closer to the eye. This condition can be inherited or associated to premature birth and may occur at any age. Divergent or negative (-) power eyeglasses are prepared to focus light rays in retina, thus improving eye vision. Myopia usually progresses annually and stabilises around the age of 20. Nowadays, no standard treatment is accepted as effective to interrupt this progression. Some recent epidemiological data suggest an association between sun exposure and myopic progression decreasing. On the other side, there is no specific evidence about excessive use of computer or portable devices as being a cause for increasing myopia in individual cases. Contact lenses can be used as an alternative to eyeglasses for those who are already capable of independent hygienic care. In the appropriate age, surgery may be an alternative if the refractive error is stable.
Astigmatism takes place when cornea is more curved at a given angle (axis) than at its opposite. The eye shape is more like a rugby ball. Prevalence of astigmatism is higher in childhood and pre-adolescence. Light rays are concentrated in different points (in front and/or behind retina) and both nearsightedness and farsightedness is affected. Cylindrical power lenses are prescribed for significant astigmatism which, if not corrected, may lead to an under optimal adult vision. Contact lenses and surgery are alternatives to eyeglasses at an appropriate age.