Diffraction limit

Summary

The diffraction limit is the finest detail that an optical system can resolve when its only flaw is diffraction at its finite aperture. A system that reaches it is called diffraction-limited: its lenses are good enough that aberrations no longer matter. For a microscope the classical Abbe limit is the period $\lambda / (2\,\mathrm{NA})$, where $\lambda$ is the wavelength and $\mathrm{NA}$ is the numerical aperture. The Rayleigh criterion gives a similar bound for two point sources.