The resolution of optical devices is limited by diffraction.
 So even the most perfect lens can't quite generate a point image at its focus, but instead there is a bright central pattern now called the Airy disk, surrounded by concentric rings comprising an Airy pattern.
The size of the Airy disk depends on the light wavelength and the size of the aperture.
John Herschel had previously described the phenomenon, but Airy was the first to explain it theoretically.

This was a key argument in refuting one of the last remaining arguments for absolute geocentrism: the giant star argument.
Tycho Brahe and Giovanni Battista Riccioli pointed out that the lack of stellar parallax detectable at the time entailed that stars were a huge distance away.
 But the naked eye and the early telescopes with small apertures seemed to show that stars were disks of a certain size.
This would imply that the stars were many times larger than our sun (they were not aware of supergiant or hypergiant stars, but some were calculated to be even larger than the size of the whole universe estimated at the time).
 However, the disk appearances of the stars were spurious: they were not actually seeing stellar images, but Airy disks.
With modern telescopes, even with those having the largest magnification, the images of almost all stars correctly appear as mere points of light.
The resolution of optical devices is limited by diffraction.  So even the most perfect lens can't quite generate a point image at its focus, but instead there is a bright central pattern now called the Airy disk, surrounded by concentric rings comprising an Airy pattern. The size of the Airy disk depends on the light wavelength and the size of the aperture. John Herschel had previously described the phenomenon, but Airy was the first to explain it theoretically.
This was a key argument in refuting one of the last remaining arguments for absolute geocentrism: the giant star argument. Tycho Brahe and Giovanni Battista Riccioli pointed out that the lack of stellar parallax detectable at the time entailed that stars were a huge distance away.  But the naked eye and the early telescopes with small apertures seemed to show that stars were disks of a certain size. This would imply that the stars were many times larger than our sun (they were not aware of supergiant or hypergiant stars, but some were calculated to be even larger than the size of the whole universe estimated at the time).  However, the disk appearances of the stars were spurious: they were not actually seeing stellar images, but Airy disks. With modern telescopes, even with those having the largest magnification, the images of almost all stars correctly appear as mere points of light.
George Biddell Airy