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Normal vision for a achromatopsic colour-blind person. Courtesy NIH National Eye Institute
The same view when achromatopsic and myopic.

Myopia, also known as nearsightedness or short-sightedness, is a refractive defect where the person affected usually can see nearby objects clearly but distant objects appear blurred. Myopia is caused by a weakening of the ciliary muscle which controls the eye's lens. The weak muscle can't adjust the lens enough to see far distances, causing far-off things to be blurred. About one quarter of the adult population in the United States has myopia.

Myopia 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. The onset of myopia is usually around the pre-teen/teenage years, and can gradually worsen through adolescence, especially if glasses are prescribed. (Corrective lenses (e.g. glasses) do the ciliary muscle's work for it, making it even weaker.) Eventually during adulthood, myopia levels off.

There is evidence of a correlation between shared genetic material and myopia, leading some to believe it is inherited (this is an example of the post hoc ergo propter hoc fallacy). However, other research disputes this. Studies done with Native American tribes found that before the introduction of widespread literacy, only the tribal records-keeper had myopia but none of the rest of the population. But after all children were taught to read, the percentage of children with myopia was approximately equal to that of the rest of America. The correlation can be explained by things like a tendency of people whose parents read a lot to read a lot themselves.

Myopia is measured in diopters, which is the combined power of the cornea and the lens needed 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.

Myopia can be cured through a variety of eye exercises, designed to stretch the ciliary muscle and bring it back to full strength. The most important of these exercises is called "pumping" and involves holding an object, like a pencil or finger, near to your face and practice switching your focus between it and an object in the distance.

Corrective lenses, - eyepieces, such as those that are variously termed spectacles or glasses, or contact lenses are also often prescribed but, as noted above, these only make the condition itself worse (although they relieve the symptoms). 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.

One 2002 study suggested that it may be caused by over-consumption of bread in childhood.