In geometrical optics, is called focus (or image point) the point in the construction of a conic section where light rays originating from a point on the object converge. The word comes from the Latin for hearth or fireplace and appears to have been first used in mathematics, in describing an ellipse, by Johannes Kepler.
Aberrations of the imaging optics may cause this non-ideal focusing. In the absence of significant aberrations, the smallest possible blur circle is the Airy disc, which is caused by diffraction from the optical system’s aperture. Aberrations tend to get worse as the aperture diameter increases, while the Airy circle is smallest for large apertures.