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The imaging system's resolution can be limited either by [[optical aberration|aberration]] or by [[diffraction]] causing [[Focus (optics)|blurring]] of the image. These two phenomena have different origins and are unrelated. Aberrations can be explained by geometrical optics and can in principle be solved by increasing the optical quality of the system. On the other hand, diffraction comes from the wave nature of light and is determined by the finite aperture of the optical elements. The [[lens (optics)|lens]]' circular [[aperture]] is analogous to a two-dimensional version of the [[Slit experiment|single-slit experiment]]. [[Light]] passing through the lens [[Interference (wave propagation)|interferes]] with itself creating a ring-shape diffraction pattern, known as the [[Airy pattern]], if the [[wavefront]] of the transmitted light is taken to be spherical or plane over the exit aperture.
The interplay between diffraction and aberration can be characterised by the [[point spread function]]<!--Maybe should go after--> (PSF). The narrower the aperture of a lens the more likely the PSF is dominated by diffraction. In that case, the angular resolution of an optical system can be estimated (from the [[diameter]] of the aperture and the [[wavelength]] of the light) by the Rayleigh criterion defined by [[Lord Rayleigh]]: two point sources are regarded as just resolved when the principal diffraction maximum (center) of the [[Airy disk]] of one image coincides with the first minimum of the [[Airy disk]] of the other,<ref>
{{cite book
|last1=Born |first1=M. |author-link=Max Born
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|doi=10.1080/14786447908639684
|url=https://zenodo.org/record/1431143
}}</ref> as shown in the accompanying photos. (In photos that show the Rayleigh criterion limit, the central maximum of one point source might look as though it lies outside the first minimum of the other, but examination with a ruler verifies that the two do intersect.) If the distance is greater, the two points are well resolved and if it is smaller, they are regarded as not resolved. Rayleigh defended this criterion on sources of equal strength.<ref name=rayleigy1879 />
Considering diffraction through a circular aperture, this translates into:
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