The authors have analyzed how much current can be obtained in the probe of an optimized two-lens focused ion beam (FIB) system. This becomes relevant, as systems become available that have the potential to image and/or fabricate structures smaller than 10 nm. The probe current versus probe size curves were calculated for a commercial gallium-FIB, the nano-FIB system, and the helium microscope, using partly published, partly estimated system parameters. The current in sub-10 nm probes in the Ga systems turns out to be limited by the reduced brightness of the source and the chromatic aberration of the objective lens. In probes larger than 40 nm the current is limited by the angular current density and the spherical aberration of both lenses. The He system is limited at all probe sizes by the angular current density of the source and the chromatic aberration of both lenses in sub-5 nm probes and the spherical aberration of both lenses at probes larger than 10 nm. As the emission current of the He source is much smaller than that of the Ga source, the statistical Coulomb interactions in the gun lens region do not contribute to the total probe size, as is the case for the Ga systems.