In
geometrical optics, a
focus, also called an
image point, is the point where
light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the
blur circle. This non-ideal focusing may be caused by
aberrations of the imaging optics. 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.