What Is a Refractive Error?
In the normal, healthy eye, the cornea is clear, smooth and round and the lens is clear, allowing light rays to pass through unobstructed and be focused sharply on the retina. However, if the cornea is not as smooth, clear or round as it should be, or the eyeball is too long or too short, light rays will bend (or refract) at odd angles, leading to blurry or distorted vision. This inability to achieve sharp focus is called refractive error, and is the most common eye problem. Refractive errors include nearsightedness, or myopia; farsightedness, or hyperopia; or astigmatism.
Similarly, the eye will have focusing problems if the naturally clear lens is cloudy due to cataract, isn’t as flexible as it should be (causing presbyopia), or has other problems with the way it functions.https://www.aao.org/eye-health/glasses-contacts-list