Myopia
Normal vision for a achromatopsic colour-blind person. Courtesy 
The same view when achromatopsic and myopic.
Myopia, also known as nearsightedness or short-sightedness, is a refractive defect of the eye, where the person affected usually can see nearby objects clearly but distant objects appear blurred.
Myopia is the most common eyesight problem in the world. About one quarter of the adult population in the United States has myopia. In places like Japan and Taiwan, as many as one in three or one in two of the adult population is myopic.
Myopia is measured in diopters; specifically, the strength of the corrective lens that must be used to enable the eye to focus distant images correctly on the retina. Myopia of 6.00 diopters or greater is considered high, or severe, myopia. People with high myopia are at greater risk of more acute eye problems such as retinal detachment or glaucoma. They are also more likely to experience floaters.
The most widely held theory of the cause of myopia is that it is mainly hereditary. Under this theory, the eye is slightly elongated front to back, causing images to be focused in front of the retina rather than directly on it. It is usually discovered during the pre-teen years between eight and twelve years of age. It most often worsens gradually as the eye grows during adolescence and then levels off as a person reaches adulthood.
Mainstream ophthalmologists most commonly correct myopia through the use of corrective lenses, such as spectacles or contact lenses. It may also be corrected by refractive surgery, such as LASIK. The corrective lenses have a negative dioptric value (i.e. are concave) which compensates for the excessive positive diopters of the myopic eye.
Another theory is that myopia is caused by a weakening of the ciliary muscle which controls the eye's lens. The weak muscle is unable to adjust the lens enough to see far distances, causing far-off things to be blurred. This theory states that the muscle's weakness is usually caused by doing lots of "nearwork", like reading books or using a computer screen. Since the eye rarely has to focus on far distances, the muscle is rarely used and, as a result, becomes weak. Since corrective lenses do the ciliary muscle's work for it, proponents of this theory suggest that they make it even weaker, increasing the problem. Instead, they recommend a variety of eye exercises to strengthen the muscle. A problem with this theory is that mainstream ophthalmology and medicine hold that the ciliary muscle is used when focussing at close distances, and is relaxed when accommodating for distant vision. Other theories suggest that the eyes become strained by the contant extra work involved in "nearwork" and get stuck in the near position, and eye excercises can help loosen the muscles up thereby freeing it for far vision.
One 2002 study suggested that it may be caused by over-consumption of bread in childhood.