Using the ion mirror image (IMIM) technique, a focused ion beam (FIB) microscope is used to investigate the charging phenomenon of Polymethyl methacrylate (PMMA). The effect of the experimental chamber’s finite size is studied using classical scattering theory. We test the widely held belief that the method tests the radius of curvature of the equipotential by performing a thorough calculation of the Ion orbits in the presence of extended sources. We show that, near to the chamber walls, the field lines bend until they are normal to the walls, the field is small, and the ion orbit is unaffected, as well as how to get rid of the “mirror effect”.