Abstract
A critical survey of the published data of the focal properties of magnetic electron lenses has revealed several inconsistencies between the findings of various authors. Some of these can be ascribed to errors in computation, but others stem from inherent limitations imposed by the conventional presentation of results in the form of `universal' curves or from the use of relative rather than fixed units, such as centimetres. The results of more accurate computations, especially of spherical aberration, are presented in a new form which is particularly suited to the determination of lenses of optimum performance. The results are applied to the question of determining the best possible objective lens for high resolution electron microscopy.