The frequency responses of eight apodized systems with circular aperture have been investigated. The apodization has been done by dividing the aperture into two and three rings of different areas either by using a π -phase distribution or by amplitude transmittance variations in the different rings. It is found that the response of the system can be improved in any desired range of frequencies by selecting its special form, its obstruction ratios and transmittances in the different rings, as the case may be.