Due to their finite inertia,particles carried in a gas follow trajectories that differ from fluid streamlines. They may thus be size-segregrated, though only with the modest resolving power allowed by the dependence of their trajectories on the fluid streamline Psi in which they are initially seeded. This limitation is greatly reduced in the present work via aerodynamic focusing, a phenomenon discovered by Israel and Friedlander and briefly reviewed here. McMurry and his colleagues have recently shown that passage through a succession of coaxial converging nozzles brings virtually into the axis of symmetry (Psi = 0) all particles within a relatively wide size range. Considerable gains in instrument resolution may thus be achieved by inserting a series of focusing nozzles right upstream of an aerosol size spectrometer. This general principle is demonstrated here for the case of variable-pressure impactors, which are ideally suited for coupling to the focusing lenses. The resolution improvements found are impressive, particularly in the viscous and compressible regimes.