Many common vision problems are caused by refractive errors. This is an irregularity in the way light passes through the eye. Normally, light enters the eye and is focused at a single point on the retina. With a refractive error, light rays do not refract or bend properly to achieve a single focus point on the retina. Instead, light rays either focus in front of the retina, behind the retina, or do not focus at a single point. Refractive errors such as nearsightedness, farsightedness, astigmatism and presbyopia usually result from a problem with the shape of the eye, cornea or lens.