Myopia, which is measured in diopters by the strength or optical power of a corrective lens that focuses distant images on the retina, has also been classfied by degree or severity.Low myopia usually describes myopia of -3.00 diopters or less. Medium myopia usually describes myopia between -3.00 and -6.00 diopters. Those with moderate amounts of myopia are more likely to have pigment dispersion syndrome or pigmentary glaucoma.

High myopia usually describes myopia of -6.00 or more. People with high myopia are more likely to have retinal detachments and primary open angle glaucoma. They are also more likely to experience floaters, shadow-like shapes which appear singly or in clusters in the field of vision.[citation needed] Roughly 30% of myopes have high myopia.

Congenital myopia, also known as infantile myopia, is present at birth and persists through infancy.[
Youth onset myopia occurs prior to age 20.
School myopia appears during childhood, particularly the school-age years. This form of myopia is attributed to the use of the eyes for close work during the school years.

Adult onset myopia
Early adult onset myopia occurs between ages 20 and 40.
Late adult onset myopia occurs after age 40.

The global prevalence of refractive errors has been estimated from 800 million to 2.3 billion. The incidence of myopia within sampled population often varies with age, country, sex, race, ethnicity, occupation, environment, and other factors. Variability in testing and data collection methods makes comparisons of prevalence and progression difficult.

In some areas, such as Japan, Singapore and Taiwan, up to 44% of the adult population is myopic. A recent study involving first-year undergraduate students in the United Kingdom found that 50% of British whites and 53.4% of British Asians were myopic.

In Australia, the overall prevalence of myopia (worse than -0.50 diopters) has been estimated to be 17%. In one recent study, less than 1 in 10 (8.4%) Australian children between the ages of 4 and 12 were found to have myopia greater than -0.50 diopters. A recent review found that 16.4% of Australians aged 40 or over have at least -1.00 diopters of myopia and 2.5% have at least -5.00 diopters.