We report on a general analytical procedure to analyse the axial focusing properties of uniform cylindrical waves truncated by a rectangular window. The resulting on-axis diffraction pattern explicitly depends on the square of the window height-to-width ratio. Depending on the value of this parameter, different kinds of axial behaviour are observed. In particular, it is found that for low values of this parameter and low Fresnel number, instead of the expected focal-shift effect, an inverse focal-shift phenomenon can appear, i.e. the maximum of the axial-irradiance distribution is displaced further away from the window.