Blurred Image - The focus, or focal point of a lens or parabolic mirror is the point onto which collimated light parallel to the axis is focused. Although the focus is conceptually a point, physically, the focus has a spatial extent, called the Airy disc, or blur circle, due to the width of the aperture of the imaging optics. The focus of an elliptic mirror is either of two points at which light from one point converges on the other. ...