Optical imaging resolves beyond the diffraction limit

Diffraction theory describes that an optical imaging system acts as a low bandpass filter selecting the low spatial frequencies of an object's spectrum. This means that the imaging system has a transfer function that multiplies the overall frequency content of the object and introduces a physical limitation by posing a cut-off frequency in the Fourier domain and as a result, limits the resolving power. This cut-off frequency value is defined by numerical aperture of the imaging lens and limits the maximum possible resolution to a value of λ/2 for air immersion optical systems and incoherent illumination. Information theory provides the theoretical background to solve this problem by establishing a theorem that states the information capacity of a imaging system remains constant. The expansion of the bandwidth is possible by encoding-decoding additional spatial frequency information onto the unused parameters of the imaging system and then it will pass through the limited system aperture.