Understanding how vision works requires a basic knowledge of how the eye focuses light. Ideally, the eye is able to perfectly focus light rays coming through the cornea onto the retina. When the curve of the cornea is too steep, too flat or irregular, light rays do not focus directly on the retina, producing impaired vision which are called refractive errors. The most common types of refractive errors are:

Nearsightedness
Nearsightedness is caused by a cornea that is too curved (too steep) or an eyeball that is too long. Light focuses in front of the retina instead of on it. If your eyes are nearsighted, you may see well up close, but distant objects will be blurry.

Farsightedness
Farsightedness is the opposite. In this case, the cornea is not curved enough (too flat) or the eyeball is too short. Light does not focus quickly enough inside the eye, so the image on the retina is blurred. If you are farsighted, you may have more difficulty seeing objects close to you than distant objects.

Astigmatism
Astigmatism often occurs in eyes that are already nearsighted or farsighted. The condition is caused by a cornea with asymmetrical curvature, where the cornea is shaped more like a football than a basketball. The eye cannot focus properly at any distance because there are multiple points of light hitting the retina in different areas. If you have astigmatism, you likely experience a ghosting, blurring, or doubling effect in your vision for both near and far objects.