Aerosol impactors generally use a collector with a diameter d(c) considerably larger than that of the nozzle d(n). Focusing impactors, however, bring all the aerosol very near the central streamline and may operate at d(c) < d(n). Some advantages of this possibility are investigated here including: (i) reduction of subcritical particle deposition rate by Brownian motion (proportional to d(c)(2)) and concomitant increase in resolution without the need for electrostatic blowing; (ii) reduction of the Stokes number S* at 50% collection efficiency, which lowers the minimum particle size that the impactor can measure. This second advantage is limited, however, because the reduction in S* hurts the efficiency of the focusing lenses, broadening the aerosol beam. When this increasing beam width exceeds the decreasing collector diameter, impactor resolution is drastically reduced. Based on measurements with mobility diameters as small as 9.7 nm, the minimum diameter of unit density spheres analyzable with this impactor is found by extrapolation to be 4.1 nm with electrostatic repulsion, and 3.13 nm without it.