The determination of the quality of an imaging system is not an easy task for, in general, at least three parameters, strictly interdependent, concur in defining it: resolution, contrast, and signal-to-noise ratio. The definition of resolution itself in scanning microscopy is elusive and the case of scanning ion microscopy is complicated by the damage of the sample under the ion beam, which, especially for small features, can be the limiting factor. This is indeed the case for most focused ion beam systems, which exploit beams of Ga+. The only way to overcome this limit is to exploit sources of low mass ions, such as H+ and He+. In this article the authors analyze the way the sputtering may affect the resolution, defined as smallest detectable feature in an image, of a scanning ion microscope, for heavy and light ions, in the case of spherical features. It appears that the fundamental limit to the resolution in scanning microscopy is not given by the spot size, but by the dynamics of the interaction of the beam with the sample and the consequent modification of the sample's geometry, even for beams of light ions. For example, in the case of Sn nanospheres under a He+ beam, the authors found a minimum theoretical detectable particle size limit of similar to 1 nm and an experimental limit of similar to 5 nm.