We present a method to calculate the electrostatic field between a metallic tip of arbitrary shape and a sample surface. The basic idea is to replace the electrodes by a set of ‘‘image’’ charges. These charges are adjusted in order to fit the boundary conditions on the surfaces. As an application of the method, we describe the field characteristics of a field‐emission diode as a function of the gap between electrodes for different tip shapes. A comparison between numerical and analytical results is presented. The results do not depend on the overall tip geometry only for gap distances smaller than ≊1/2 the tip radius. The field enhancement factor due to the presence of small protrusions on the tip apex is calculated and their influence in near‐field‐emission scanning tunneling microscopy is also discussed. We show that the electron‐field emission from the sample is stable against tip‐shape changes due to adsorbate diffusion or atomic rearrangements. © 1996 American Institute of Physics.