The complete field plot obtained by ``relaxation'' was used to determine electron trajectories for parallel incidence at heights up to one‐third the bore diameter, from a numerical integration of the trajectory equation using the method of differences. The accuracy attained in this method yields cardinal points to within one percent. It was found that the focal lengthfnfor a ray of heightrnwas given byfn=f0−brn2, wherebis a constant; also, because of the curvature of the principal planes, the distancednto the midplane follows a lawdn=c−drn2wherecanddare constants. A consequence of this is that for the same variation in focal length, the greater the curvature of the principal plane the larger the spherical aberration of the lens.The trajectory tracings indicate that spherical aberration is small at heights up to one‐twelfth the bore diameter, but increases with height till at a value of one‐third the bore, the percentage change in focal length has decreased by some 40 percent. Confirmation of focal properties was obtained using the lens in a single stage electron microscope and examining the pincushion distortion in the image of a gauze object.Evaluation of the spherical aberration using the weak lens approximation of Scherzer leads to gross errors both in magnitude and variation, e.g., the aberration does not remain constant with voltage ratio. The experimental data with the electron microscope indicate little or no change in aberration with increase in the power of the lens; this does not agree with the results of the mathematical analysis of Ramberg.