Refractive errors in ophthalmology include myopia, hyperopia, and astigmatism. The optical principle behind these is that parallel light rays, after passing through the optical system of the eyeball, do not focus on the retina, and therefore do not form a clear image. This condition is known as a refractive error. In everyday life, some people mistakenly believe that presbyopia is also a refractive error, but presbyopia is not a refractive error; it is a physiological condition. The common refractive errors are the three types I just mentioned: myopia, hyperopia, and astigmatism.