Astigmatism produces blurred vision at all distances. Most astigmatism is caused by the shape of the front surface of the eye (the cornea). It can also be caused by slight tilting of the lens inside the eye. It may be an inherited characteristic or a normal variation accompanying growth.
Imagine having an eyeball the shape of a rugby ball rather than a soccer ball. As the light passes into the eye, the light rays are bent to differing degrees resulting in a distorted image presented at the retina. In some directions the image will be more out of focus than others.
Even slight degrees of astigmatism may cause headaches, fatigue and reduce concentration. This is because the eyes may try without success to correct the blur, and because there is a tendency to screw up the eyes to try to see better. This can produce discomfort in the muscles of the eyelid and face.