In an ideal world, the surface of the cornea (the transparent layer of tissue at the front of the eye) would be perfectly spherical in shape, and incoming light would form a single focus, ideally on the retina at the back of the eye.
However, in the real world the cornea is often irregular in shape and often conforms to the shape of a rugby ball. This is because light rays are focused on more than one place in the eye so you don't have one clear image. For instance it may make it hard to tell 'N' from 'H'. Astigmatism is common amongst both long-sighted and short-sighted people. Vision without glasses can be blurred and slightly distorted at all distances.