One of the eye problems that causes blurred vision is astigmatism. This happens when the cornea, or “window of the eye”, is out of shape. A normal cornea has a perfect curve, with the same degree of slope on all sides. Light rays that pass through the cornea come together at a single point on their way to the retina.
This simply isn’t possible in the astigmatic eye. The rays of light don’t come together at the same point. There are two separate images. This doesn’t mean that people with astigmatism see double, but it does blur their vision.