or an imaginary plane such as the first principal plane or the entrance pupil plane. In diopters, spherical refractive errors can be expressed as K=1/k, where k is the distance in meters from the reference plane to an eye’s far point, and K is the refractive error in diopters. Thus, a person with myopia would have a negative refractive error, a person with emmetropia would have zero refractive error and a person with hyperopia would have a positive refractive error. In the case of regular astigmatism, refractive error needs to be expressed as 3 values: classically as sphere, cylinder and axis. However, it can also be expressed in vector terms, for example, M (mean sphere), J0 (With the rule/Against the rule astigmatism), J45 (oblique astigmatism). Refractive errors containing higher order aberrations (sometimes referred to as irregular astigmatism) can be expressed for a given pupil size using wavefront errors or optical path differences, often as coefficients for Zernike polynomials. A more subjective quantity visual acuity (expressed as a fraction) may be used, but there is no direct or exact conversion between the two. Risk factors Genetics There is evidence to suggest genetic predilection for refractive error. Individuals that have parents with certain refractive errors are more likely to have similar refractive errors. The Online Mendelian Inheritance in Man (OMIM) database has listed 261 genetic disorders in which myopia is one of the symptoms. Myopia may be present in heritable connective tissue disorders such as: Knobloch syndrome (OMIM 267750); Marfan syndrome (OMIM 154700); and Stickler syndrome (type 1, OMIM 108300; type 2, OMIM 604841). Myopia has also been reported in X-linked disorders caused by mutations in loci involved in retinal photoreceptor function (NYX, RP2, MYP1) such as: autosomal recessive congenital stationary night blindness (CSNB; OMIM 310500); retinitis pigmentosa 2 (RP2; OMIM 312600); Bornholm eye disease (OMIM 310460). Many genes that have been associated with refractive error are clustered into common biological networks involved in connective tissue growth and extracellular matrix organization. Although a large number of chromosomal localisations have been associated with myopia (MYP1-MYP17), few specific genes have been identified. Environmental In studies of the genetic predisposition of refractive error, there is a correlation between environmental factors and the risk of developing myopia. Myopia has been observed in individuals with visually intensive occupations. Reading has also been found to be a predictor of myopia in children. It has been reported that children