The advantages of a retarding field immersion lens for focusing low energy electrons and ions have been well chronicled. One recent development is a retarding field lens in which the sample is outside the retarding field (i.e., a nonimmersion lens). Thus, any sample roughness will not distort the focusing field. We describe here a simple model and analysis that indicates that the performance of such a lens can be almost as good as that of a retarding field immersion lens. For example, it should be possible to focus 1 keV electrons with an energy spread of 1 eV into a beam diameter of 4 nm using realistic values of maximum voltage and retarding field strength; the effective resolution under these conditions is estimated to be 1.4 nm.