Traditional scalar light fields have spatially uniform polarization states, whereas vector light fields have spatially varying polarisation states compared to scalar light fields. The spatial variation of the polarization characteristics enables the vector light field to have greater control freedom with potential applications in the field of focal field design, optical micromanipulation, micromachining, and quantum information. In this paper, based on the Richards-Wolf vector diffraction theory, the effects of variable aperture as well as lens numerical aperture on the focusing characteristics of a double-ring-shaped azimuthally polarized beam are investigated based on the Richards-Wolf vector diffraction theory. The results clearly show that by changing the aperture size and lens numerical aperture, it is possible to change not only the focal spot intensity and the focal spot size, but also the length of the dark channel in the focal field, resulting in longer dark channels. The resulting variable focal field dark channel can be used in the future for optical micromanipulation and microprocessing applications.