| Summary: | In this paper, the problem to estimate the on-axis resolution in creating a desired field profile by radiation of an aperture <i>A</i> is addressed. The latter applies in both diffractive optics and antenna synthesis. This is because the ‘aperture theory’, <i>A</i> can schematize a source, for example, an antenna or a lens illuminated by an incident field radiating a significant field only on the same aperture. The analysis refers to a customary axicon geometry consisting of a circle aperture transverse to the observation domain. The aim was to find a resolution formula allowing to highlight the impact of aperture geometrical parameters for configurations that are below the Fresnel approximation. The results show that the aperture cannot approximate the target field with the same level of accuracy along with the observation domain. In particular, near the aperture, smaller details can be retrieved and as the distance increases this ability degrades.
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