Astigmatism

The astigmatic eye is unable to focus the entire image on the retina because of the "egg-shaped" curve of the cornea on the front of the eye. Rays entering the "North-South" curve of the cornea may focus correctly on the retina while those rays entering the "East-West" curve may not.

Most people have a small amount of astigmatismy. If they have more than a small amount it does effect vision, usually both at far and near.