In reality the focus is never a point but a small spot called the blur circle.
An image, or image point or region, is in focus if light from object points is converged almost as much as possible in the image, and out of focus if light is not well converged.
- "Standard Microscopy Terminology". University of Minnesota Characterization Facility website. http://www.charfac.umn.edu/glossary/f.html. Retrieved 2006-04-21.
- 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.