Your eye’s main function is to focus light. Common visual eye problems are diagnosed based on how your eye does just that. Nearsightedness, Farsightedness, and Astigmatism are measured in diopters and these diopters describe the light bending properties of an optical system.
Nearsighted, also known as myopia, means that you can see objects that are closer to you clearer than you can see objects that are far away. The more nearsightedness you are, the most blurred your distant vision becomes. When your eye is too long in relation to your cornea, the incoming light focuses in front of the retina, this is what causes your vision to be blurred.
Farsighted, also known as hyperopia, means you can see object far away clearer than you can see them up close. Unlike nearsightedness where you eye is too long, in farsightedness your eye is too short in relation to your cornea. Since your eye is too short, the incoming light cannot bend enough to focus on the retina.
Astigmatism means your cornea is shaped like an egg. Since your cornea is shaped like an egg, you have more than one focal point and this distorts what you see. Many patients who are nearsighted and farsighted may also have astigmatism. If you have a high degree of astigmatism, your visions is blurred for both near and far objects.