diffraction limit

Noun

 * 1)  The minimum angular separation of two sources that can be distinguished by a telescope (depending on the wavelength of the light being observed and the diameter of the telescope).
 * For telescopes with circular apertures, the size of the smallest feature in an image that is diffraction limited is the size of the Airy disk. As one decreases the size of the aperture in a lens, diffraction increases. At small apertures, such as f/22, most modern lenses are limited only by diffraction.