Astigmatism is a very common, yet often misunderstood condition. Like nearsightedness and farsightedness, astigmatism is a refractive error, not an eye disease, and is easily treated with eyeglasses or contact lenses. A refractive error means that the eye cannot focus light on to the retina properly. In a normal eye without refractive error, the light passing through the cornea hits directly onto the retina.
In a perfectly shaped eye, the focusing elements (cornea and lens) have a flawlessly smooth curvature, causing light to focus in one point either in front (nearsightedness) or behind (farsightedness) the retina.
Nearsightedness is when the light falls in front of the retina due to a more rounded cornea. A lens with minus power will allow the light to fall onto the retina.
Farsightedness is when the light falls behind the retina due to a flatter cornea. A lens with plus power will allow the light to fall onto the retina.
Astigmatism occurs when your cornea or lens isn’t perfectly round causing light rays focus onto multiple points.