This article presents a novel electrostatic lens which is particularly advantageous in producing a smaller spot focused ion beam (FIB). The lens has a tandem structure made up of conventional einzel lenses operated in the acceleration (accel) mode. The structure allows the lens to work under significantly lower excitation voltages compared with conventional accel lenses, making it possible to converge high energy FIB at short focal lengths to minimize aberrations. The minimum spot size obtainable for a 30 keV FIB with 1 pA current has been calculated for various objective lens applications under a restriction where the excitation voltage should be below the beam voltage (30 kV). Spot sizes are 7.3, 6.6, and 4.8 nm for the conventional accel, decel, and tandem accel lenses discussed in this article, respectively.