Refractive error refers to the inability of external light to focus on the macula in the fundus to form a clear image after passing through the eye, including myopia, hyperopia, and astigmatism. In myopia, the focal point is in front of the retina, in hyperopia, the focal point is behind the retina, and in astigmatism, the focal point is not formed, but rather a focal line. The eye is like a fine camera, the negative is equivalent to the fundus retina, if the light cannot be focused on the negative, the photo will not be clear.