Normally, light in all planes is focused correctly on the back of the eye ... ... producing a clear, sharp image.

This is because the lens in the eye is perfectly round. A lens which is 'Astigmatic' is shaped like a rugby ball.

This means that light in different planes has different focal points ...

... resulting in objects in some planes being out of focus.

The degree of Astigmatism can vary from eye to eye.

This condition is particularly noticeable when objects have straight lines.