The invention relates to a method for determining lens errors in a Scanning Electron Microscope, more specifically to a sample that enables such lens errors to be determined. The invention describes, for example, the use of cubic MgO crystals which are relatively easy to produce as so-called `self-assembling` crystals on a silicon wafer. Such crystals have almost ideal angles and edges. Even in the presence of lens errors this may give a clear impression of the situation if no lens errors are present. This enables a good reconstruction to be made of the cross-section of the beam in different under- and over-focus planes. The lens errors can then be determined on the basis of this reconstruction, whereupon they can be corrected by means of a corrector.