  This paper describes theoretical modelling of electrostatic lenses based on
3, 4 and 5 closely spaced cylindrical electrodes, respectively. In each case,
modelling is carried out numerically using commercial packages SIMION and
LENSYS, and a variety of performance parameters are obtained. These include the
magnification, the 3rd order spherical and chromatic aberration coefficients.
Special cases such as zoom lens (i.e., lenses whose magnification may be
changed without losing focus) are considered. Results are obtained as a
function of the ratios of the electrode lengths and gaps, and as a function of
ratios of the controlling voltages.
  As a result, it is shown that how a multi-element lens system can be operated
with the whole focal properties in a useful mode for using in experimental
studies.
