Thickness variation of visually opaque foils used as x‐ray and ultraviolet (XUV) photon filters and photocathodes can introduce anomalies in recorded intensities, images, and spectral line profiles. In laser fusion studies, mass thickness variations of laser‐accelerated foils can hydrodynamically disrupt their motion. We have used transmitted electrons to obtain quantitative local (area ≤1 &mgr;m2) measurements of mass thickness as well as qualitative images of mass thickness variations in free‐standing Be, C, and Al foils presently in use in the above areas of investigation. Substantial mass thickness variations, up to a factor of 4 for Al and 1.3 for Be, were found which can produce considerable modulations in transmitted XUV photon intensity.