A study has been made to compare theoretical predictions of the performance of an optical column using a liquid metal ion source with experiment. Rather than estimating the beam size by calculating disks of confusion due to the chromatic and spherical aberrations, calculations of the current density profile were carried out using a new method which determines trajectories between object and image planes using the geometrical aberration theory. The trajectories are then used to find the current density which was then integrated to simulate the beam passing over a knife edge, so the rise distance could be estimated. It is shown that the focus conditions for optimum resolution of the beam may differ significantly from the conditions which optimize the rise distance of the beam. An experiment was then carried out to measure the actual rise distance for a variety of operating conditions of the actual ion gun. Good agreement was found between experiment and calculation.