Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. The people who catch the astigmatism will have the following symptoms: vision loss;Visual fatigue;always Narrowing eyes when look at things. Astigmatism can caused double vision due to light into the eyes can not focus a point. Astigmatism can be corrected by wearing the right astigmatic lens. If you friend is older enough and the situation is serious, advise him to do corneal refractive surgery.
Astigmatism is a kind of refractive error status, with the cornea and lens surface bending don't agree about. The light cannot accurately focus on the retina and form the clear objective image. This kind of situation is called astigmatism. Only in the situation of unstable eye pressure can the ones with astigmatism cause the double vision.