Myopia (short-sightedness) occurs when the eye is too long from front to back or the cornea is too steep, causing incoming light rays to focus in front of the retina instead of directly on it. Because the focal point lies anterior to the retinal surface, distant objects appear blurred while near objects remain clear. The elongated axial length increases the optical power of the eye relative to its focusing system. Glasses, contact lenses, or laser vision correction work by shifting the focal point backward so that light is brought onto the retina. In high myopia, the excessive axial length can also increase the risk of retinal and macular complications. 

Hyperopia (long-sightedness) occurs when the eye is shorter than normal from front to back or the cornea is too flat, reducing the eye’s focusing power. As a result, incoming light rays are brought to a focus behind the retina rather than directly on it. This makes near objects appear blurred, while distant objects may be clearer, especially in younger people who can compensate by using their focusing muscles. The eye must work harder to shift the focal point forward onto the retina, which can cause eye strain and headaches. Optical correction moves the focal point forward so images are focused on the retinal surface. 

Normal corneas are round like a soccer ball.  Astigmatic corneas have curved shaped like an American football or rugby ball.  Due to the uneven curvature of the cornea, light rays entering the eye are focused on multiple points instead of a single point on the retina. This optical mismatch causes images to be blurred or distorted at both near and far distances.