<unk> , also known as refraction<unk> , is a problem with focusing light accurately on the retina due to the shape of the eye and or cornea. The most common types of<unk>  are near-sightedness, far-sightedness, astigmatism, and presbyopia. Near-sightedness results in far away objects being blurry, far-sightedness and presbyopia result in close objects being blurry, and astigmatism causes objects to appear stretched out or blurry. Other symptoms may include double vision, headaches, and eye strain. Near-sightedness is due to the length of the eyeball being too long, far-sightedness the eyeball too short, astigmatism the cornea being the wrong shape, and presbyopia aging of the lens of the eye such that it cannot change shape sufficiently. Some<unk> s occur more often among those whose parents are affected. Diagnosis is by eye examination.<unk> s are corrected with eyeglasses, contact lenses, or surgery. Eyeglasses are the easiest and safest method of correction. Contact lenses can provide a wider field of vision; however they are associated with a risk of infection.<unk>  surgery permanently changes the shape of the cornea. The number of people globally with<unk> s has been estimated at one to two billion. Rates vary between regions of the world with about 25% of Europeans and 80% of Asians affected. Near-sightedness is the most common disorder. Rates among adults are between 15-49% while rates among children are between 1.2-42%. Far-sightedness more commonly affects young children and the elderly. Presbyopia affects most people over the age of 35. The number of people with<unk> s that have not been corrected was estimated at 660 million (10 per 100 people) in 2013. Of these 9.5 million were blind due to the<unk> . It is one of the most common causes of vision loss along with cataracts, macular degeneration, and vitamin A deficiency.
Refractive error
Instrument myopia
Cataract
Ectopia 
Corneal ectatic disorders
Congenital cataract
Auditory fatigue
Ocular hypertension