Different methods such as axilens and binary-phase filter have been investigated to improve the depth of focus. A method is proposed to calculate an amplitude-phase pupil filter and obtain the desired distribution of intensity along the optical axis. It produces a narrow spot with a uniform intensity level over a large depth of focus, comparable to the performance obtainable with binary-phase filters. This filter is of particular interest for applications where very low intensity fluctuations along the focus range are required.