[en] A method is described for measuring the intensity distribution of the electron source in a scanning transmission electron microscope (STEM) fitted with an objective lens aberration corrector. The method is applied to a C_s-corrected 300 kV field emission gun TEM/STEM, which is found to have an effective source size of 0.56 A full width at half maximum (FWHM) under optical conditions suitable for high resolution STEM imaging. This corresponds to a probe intensity distribution at the specimen plane of 0.72 A FWHM using a probe-forming aperture of 25 mrad and including the measured residual lens aberrations