![]() ![]() When the aperture is stopped down, the airy disk generated by each point light source grows, to the point where the outer rings of each airy disk begin to merge. At a wide aperture, two point light sources imaged by a sensor may only affect single neighboring photosites. Another way to look at it is when the airy disks from two point light sources resolvable by the sensor begin to merge. The diffraction limit is the point where airy disks grow large enough that they begin to affect more than a single photosite. When the airy disk grows in size and intensity as a lens is stopped down, the airy disk affects neighboring photosites. This is due to the fact that a smaller photosite covers less of the airy disk area than a larger photosite. A sensor with smaller photosites, or film with smaller grain, will have a lower limit of diffraction than those with larger photosites/grains. ![]() The "limit" of diffraction is a function of the imaging medium. As noted above, lenses are always creating a diffraction pattern, only the degree and extent of that pattern changes as the lens is stopped down. It should also be clearly noted that the diffraction limit is not actually a limitation of a lens. You realize that every one of those points of light, when focused by your lens, is generating its own airy disk on the imaging medium. The size of the airy disk, and the proportion of the disk that comprises the outer rings, and the amplitude of each wave in the outer rings, increases as the aperture is stopped down (the physical aperture gets smaller.) When you approach photography in the way Whuber mentioned in his answer: First, diffraction always happens, at every aperture, as light bends around the edges of the diaphragm and creates an " Airy Disk". There have been some very good answers, however there are a couple details that have not been mentioned. ![]()
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