Emmetropia
If you are emmetropic it means you can see well at a distance when your eyes are at rest (when they are not accommodating or trying to focus). Your eye is correctly shaped so that the light rays focus (or refract) on the retina in the back of the eye as they should. This allows for a clear image to be processed by the optic nerve/brain and seen through the eyes.

Nearsightedness
If you are nearsighted it means that you can see well up close, but things at a distance appear blurry. Nearsightedness occurs when the cornea (the front of the eye) is too steep, or the eye is too long, and the light rays focus in front of the retina, rather than on the retina as they should. This condition can be corrected by glasses, contacts, or laser vision correction.

Farsightedness
If you are farsighted it means that you might see well at a distance by focusing the eyes, but as focusing diminishes with time both near and distance vision become blurry. Farsightedness occurs when the cornea (the front of the eye) is too flat or the eye is too short, and the light rays focus behind the retina, rather than on the retina as they should. This condition can be corrected by glasses, contacts or in some cases laser vision correction.

Astigmatism
If you have astigmatism it means that your eye is not round, rather it is much like a football, which causes some light rays to focus on the retina and some to focus in front or behind the retina. This visual distortion can produce shadows, double images, or ghosting. Often people will have astigmatism combined with nearsightedness or farsightedness. This condition can be corrected by glasses, contacts or in most cases laser vision correction.