In the researching field of optical superresolution, a phase or amplitude filter is often utilized. They are easy to build, and
simple to insert to a system. A new idea on design a continuous phase filer is proposed. This type of filter is based on the
law that the focal size is mainly decided by the marginal rays in the aperture. A quadric function consisted of tangent
value is built. Assuming that the radially polarized light incident, passing through a phase filer and being focused by a
lens, the intensity distribution on the focal plane is simulated. The vector diffraction theory is used here. To get a smaller
focus, the parameters in the phase function are optimized in Matlab. The results show the superresolution effects.
Compared with others, the filters maintain higher energy utilization rate. A good prospects for application can be