Abstract: A formula of the axial irradiance distribution of a binary annular aperture is derived in detail by using the Rayleigh-Sommerfeld diffraction integral that is valid to a few wavelengths of the aperture. We calculated and analyzed the axial irradiance distribution throughout the whole space behind the binary annular aperture. The results show that its maximum depth of modulation of the oscillation is sixteen times the incident irradiance and four times that of the circular or annular aperture,and that the optical sectioning strength in the near field is better than in the Fresnel region.In the very near field, the axial irradiance distribution has a variety of subtle effects as changing its aperture parameters. These are of potential interest in photonics and fiber-optics application.