 The resolution of an optical imaging system, a microscope, telescope, or camera, can be limited by factors such as imperfections in the lenses or misalignment. However, there is a principal limit to the resolution of any optical system, due to the physics of diffraction. An optical system with resolution performance at the instrument's theoretical limit is said to be diffraction limited. The diffraction-limited angular resolution of a telescopic instrument is proportional to the wavelength of the light being observed, and inversely proportional to the diameter of its objectives entrance aperture. For telescopes with circular apertures, the size of the smallest feature in an image that is diffraction-limited is the size of the area disc. As one decreases the size of the aperture of a telescopic lens, diffraction proportionately increases. That small apertures, such as F-22, most modern lenses are limited only by exaggerated diffraction and not by aberrations or other imperfections in the construction. For microscopic instruments, the diffraction-limited spatial resolution is proportional to the light wavelength, and to the numerical aperture of either the objective or the object illumination source, whichever is smaller. In astronomy, the diffraction-limited observation is one that achieves the resolution of a theoretically ideal objective in the size of instrument used. However, most observations from Earth are seen limited due to atmospheric effects. Radial telescopes on the Earth work at a much lower resolution than the diffraction limit because of the distortion introduced by the passage of a light through several kilometers of turbulent atmosphere. Some advanced observatories have recently started using adaptive optics technology, resulting in greater image resolution for faint targets, but it is still difficult to reach the diffraction limit using adaptive optics. Radial telescopes are frequently diffraction limited, because the wavelengths they use from millimeter to meter are so long that the atmospheric distortion is negligible. Space-based telescopes such as Hubble, for a number of non-optical telescopes always work at their diffraction limit, if their design is free of optical aberration. The beam from a laser with near ideal beam propagation properties may be described as being diffraction limited. The diffraction-limited laser beam, passed through diffraction-limited optics, will remain diffraction limited, and will have a spatial or angular extent essentially equal to the resolution of the optics at the wavelength of the laser.