The optical properties of "non-sandwich-structured" zonal pupil filter based on birefringent crystal are studied in focusing optical systems with low numerical aperture. The exact pupil function of the new filter is presented by taking glass and birefringent crystal for the inner and outer zone respectively as an example. Numerical simulations of its optical properties are carried out with scalar diffraction theory, which shows that tunable transverse superresolution properties as well as extended axial focal depth can be obtained by rotating the pupil filter. This kind of filter has higher light utilization ratio than the existed pupil filters based on birefringent crystals, and its tunable ability can provide more flexibility in practical applications.