The aberrations coefficients are derived for electrostatic systems that are axially symmetric except for the presence of a localized potential defect (i.e., a patch field). The near-axis aberrations caused by such a defect near the edge of an aperture are then investigated. When a beam fills the aperture the resulting aberrations cannot be characterized in terms of coefficients, and so a new characterization of the aberrations is defined. Numerical evaluations are carried out for representative types of potential defect. Relationships are presented that enable the increase in spot size of a focused beam to be deduced from the measured or estimated mean deflection of the spot. ©1998 American Institute of Physics.