The point at which the center of each airy disk merges is the limit of resolution, and you will no longer be able to resolve any finer detail regardless of the aperture used. This is the point where a sensor is "diffraction limited", since individual point light sources no longer resolve to a single photosite.they are merging and covering more than one photosite. 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". The spread of the energy outside the central disk can cause disturbing crosstalk into adjacent detector elements in an array.There have been some very good answers, however there are a couple details that have not been mentioned. Including the energy of a diffraction blur twice in diameter of the central disk (the measure of the second dark ring) increases the total encircled by only 7%. The trend to decrease the pixel sizes in focal plane arrays to increase the system's resolution requires awareness of this limit set by nature. The diameter of this central disk isįor a LWIR system operating at a wavelength of 10 μm and an f-number of 2, the diffraction blur with 84% of the energy is 48.8 μm (approximately 0.002 in.) in diameter. If the aperture of the lens is circular, approximately 84% of the energy from the energy from an imaged point is spread over the central disk surrounded by the first dark ring of the Airy pattern. (3.74), the diffraction effect can often become the limiting factor for infrared systems. Since this blur size is proportional to the wavelength, as indicated in Eq. Its cross section and its appearance are shown in the figure below.Īiry disk, energy distribution and appearance. This is the diffraction blur or Airy disk, named in honor of Lord George Biddel Airy, a British mathematician (1801–1892). The result is that the image of a point is a blur, no matter how well the lens is corrected. However, due to the wave nature of radiation, diffraction occurs, caused by the limiting edges of the system’s aperture stop. An ideal optical system would image an object point perfectly as a point.
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