When a focused electron beam is deflected by a transverse magnetic field through a small angle, the spot shape on the target is distorted. The electron density distribution within it also varies. A novel method of calculating and presenting the characteristics of the distorted spot is presented. The positions of the electrons at the incoming aperture are mapped into the target plane subsequent to the deflection. The resultant density distribution is plotted by the computer. © 1967 The American Institute of Physics.