Magnification variations in reflection electron microscopy using diffracted beams

In reflection electron microscope images obtained using electron beams diffracted at a small angle from the surfaces of bulk specimens, it is observed that the magnification in directions almost parallel to the incident beam appears to increase rapidly with distance from the in-focus position in both directions. An explanation for this effect is offered in terms of the curvature of the lines of energy flow around the cross-over of the electron beam formed by the condenser action of the fore-field of the objective lens.