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.
Defocus is the aberration in which an image is simply out of focus. This type of aberration generally occurs using optical microscopes, telescopes, or binoculars.
Optically, defocus refers to a translation of the focus along the optical axis away from the detection surface. In general, defocus reduces the sharpness and contrast of the image. What should be sharp, high-contrast edges in a scene become gradual transitions.
Fine detail in the scene is blurred or even becomes invisible. Nearly all image-forming optical devices incorporate some form of focus adjustment to minimize defocus and maximize image quality.