A focusing impactor is used to study the influence of particle charge q on the capture efficiency versus Stokes number curve eta(S). For highly charged particles, image attraction to the collector leads to considerable particle capture even at S = 0. This produces long subcritical tails in the eta(S) curve (poor sizing resolution) and reduces the value S* of S at which eta = 1/2. A similar behavior, now due to Brownian motion, arises for singly charged ultrafine particles, with a negligible influence of the image force. Both types of tails are reduced or eliminated by a repulsive electric field E between the collector and the nozzle, though at the price of increasing S*. For repelling fields E large enough for the subcritical tails to disappear, S* is a function only of the ratio ZE/U between the electrical and hydrodynamic velocities of the particles. The functional dependence S*(ZE/U) is characterized experimentally for a nearly incompressible thin-plate orifice nozzle flow at Reynolds number Re = 68. Calibration aerosols include singly or doubly charged oil droplets with diameters between 16 and 155 nm. Also polystyrene latex spheres (PSL) 74 nm in diameter generated from a water suspension by electrospray, whose charge q was narrowly controlled in the range 540 > q/e > 390 with a differential mobility analyzer. The analysis of the rate of deposition in the stagnation point region in the limit S = 0 predicts the repulsive fields required to offset the tails, in fair agreement with those observed both for singly and doubly charged as well as highly charged particles. The conclusion is reached that impactors whose detector is an electrometer can attain rather high resolutions in all the range of sizes and charges explored.