This paper deals with an experimental procedure to illustrate the problems introduced by ferromagnetic hysteresis present in magnetic lenses of electron microscopes. The magnetic flux density is not available as a measurable quantity. Hysteresis expresses itself in the relation between the input current applied to the lens-coil and the level of sharpness (defocus) of the resulting images. The familiar hysteresis loops are not available, instead we measure hysteresis in the so-called butterfly

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... presentation. The input-profiles are non-periodic and illustrate the difficulties with reproducibility in microscopy applications. Image based feedback control is impossible since 99% of the input range yield unusable images. Analysis of the experiments is carried out using a qualitative model consisting of an interconnection of hysteresis representing the magnetic lens and a nonlinear function representing electron optics. Because this model introduces the intermediate magnetic field variable, it is possible to reconstruct and explain the results observed in the experiments.