Pupil filters can be used to overcome the limits in axial resolution imposed by diffraction in imaging systems. Axial behavior of filters is important for depth resolution in confocal microscopy. Phase filters have the advantage of improved efficiency by virtue of allowing more light to be transmitted, reduced tolerance errors, and in some case increased super resolving performance. In this paper we present a theoretical investigation of phase only filters. In addition, we describe the generation of a phase only filter from an amplitude filter using an optically-addressable liquid crystal spatial light modulator (LCSLM).