is_about refractive error eye amount measurement refractive error measurement true Dani Welter quantification of some aspect of refractive errors, which occur when the shape of the eye prevents light from focusing directly on the retina. The length of the eyeball (longer or shorter), changes in the shape of the cornea, or aging of the lens can cause refractive errors. Quantification of an individual's refractive error allows the establishemnt of a number of corrective parameters such as spherical equivalent, lens decentration and transposition