Focused ion beam (FIB) machines are becoming an accepted part of the semiconductor industry. They are used in the repair of photomasks and X-ray masks, for direct modification of devices, for failure analysis, and for process verification. As the scale of the lithography shrinks, the demands on the FIB tool increase accordingly, both in terms of its accuracy (for repair and modification) and its resolution (for imaging). One key factor that affects these parameters in the FIB column itself, in terms of its spot size performance. Many of today's applications demand spot sizes as low as 15 nm, at beam currents of 10 - 20 pA. This paper proposes a figure of merit for FIB columns, based on the optimum magnification model, that provides the column designer with a consistent technique for assessing column performance. The figure of merit can be used either as a simple metric for the final column design, or more usefully as an aid to the designer to provide feedback on how to improve column performance.