When eye focusing is not perfect, nearsightedness and farsightedness occur.
Nearsightedness (called myopia) is present when a person is able to see objects close to him well but has difficulty seeing far away objects. Light rays become focused in front of the retina. Possible causes of this include eyeballs that are too long or lens systems with too much power to focus. A concave lens can correct nearsightedness. This lens causes light to diverge slightly before it reaching the eye.
When farsightedness (called hyperopia) is present, a person is able to see far away objects well but has difficulty seeing close up objects. Light rays become focused behind the retina. Possible causes include eyeballs that are too short or by lens systems that have too little focusing power. Farsightedness can be corrected with a convex lens.
Vision also can be distorted by astigmatism, an uneven curvature of the cornea. A special lens must be shaped to correct the unevenness.