In optics, astigmatism is a lens or mirror defect in which the size and shape of an image vary for different points of focus. Light passing through different parts of an astigmatic lens, for example, is focused at different distances beyond the lens, so that the image of a point can appear variously as a short horizontal or vertical line or an ellipse. The best focus is a small circle known as the circle of least confusion.
Astigmatism in human vision
Astigmatism in human vision happens when the front part of the eye – the cornea – (or sometimes the eye lens itself) is not perfectly regular and symmetrical in shape. The astigmatism of the eye is a type of refractive error in which the eye does not focus light evenly on the retina. This results in a distorted or blurred vision at all distances. If the cornea is more oval than round (like the back of a spoon), then light passing through it isn’t focused properly on the retina at the back of the eye. This results in a blurring of vision at all distances.