How is a refractive error measured and what is a diopter?
When you read a prescription, a minus sign means nearsightedness and a plus sign means farsightedness. The first number in the prescription indicates the amount of myopia or hyperopia you have. If there are numbers written in the next two columns, these indicate the amount of astigmatism, if any, and the axis where it is present. The unit to measure a refractive error is called a diopter or D. For example, if your prescription says -2 or -2D or -2 sph or -2 sphere, it means that you are nearsighted and require 2 diopters of correction. A diopter is a unit of measurement related to the focal length of an optical system including the human eye. The larger the diopter the shorter the focal length.
