The scanned field performance of probe forming systems has been investigated leading to various methods for increasing the current that may be delivered into an electron probe scanning a square field of104lines. Programs for evaluating the third-order aberrations of both lenses and single and double deflection systems have been developed. These have been used to evaluate the performance of a typical pre-lens double deflection probe forming system. It is shown that it is possible to produce much better performance using post-lens deflection coils. Distortion, field curvature and astigmatism can be eliminated, using suitable electronics for dynamic correction, and the remaining third-order aberrations may be reduced by careful design of the deflection coils.