Demagnifying EPS have the capacity to generate as much as 5000 line pairs per diameter at a resolution as small as 0.1 μ. The diameter of the recorded image and the maximum number of line pairs per diameter decrease with increasing values of the chromatic field aberrations of the lens system, and with increasing relative energy width of the electron beam. The optimum aperture yielding maximum probe current at a given resolution and image field diameter depends mainly on the value of the coefficients of image curvature and isotropic and anisotropic astigmatism of the lens system.