Optical resolution
= Optical resolution
<Optical resolution> is the ability of an imaging system to distinguish spatially separated features under specified observation conditions. A finite <numerical aperture> spreads a point object into a <point spread function>; neighboring point images overlap. A criterion such as the <Rayleigh criterion> assigns a separation to this overlap rather than marking a sharp universal boundary between visible and invisible detail.<Resolution by Michael W. Davidson>[Formulas 1–3 and discussion of the illumination and criterion dependence]{ref}
A practical test images two small sources at known separations, keeping exposure and display <image contrast> fixed. Decreasing their separation until the chosen criterion fails measures <lateral resolution>; moving them along the imaging axis tests <axial resolution>. <Magnification> alone cannot narrow the specimen-space <point spread function>.