It is found that large radial and azimuthal distortions are generated if the geometric conditions defined for the symmetric magnetic doublet lens system are not met for the inner and outer radii of the lens casing. These distortions are found by trajectory calculation and by computing third-order aberration coefficients. When the inner radius of the object side lens casing is decreased, then pincushion distortion is generated. When the outer radius of the cylindrical object side lens casing is decreased, barrel distortion is generated. Anisotropic distortion is generally present, changing sign when the inner radius of the casing is decreased significantly. The radial distortion can be compensated by adjusting the inner and outer radii of the lens casing, whereas the azimuthal distortion cannot be compensated by such adjustment. The origin of these distortions can be well understood by studying the magnitude of the maxima and minima of the axial magnetic field and magnetic vector potential. Field curvature, astigmatism, coma radius, coma length, axial chromatic, and spherical aberrations are not influenced by such variations of the inner or outer radii of the lens casing.