Ultrafast low-energy electron diffraction holds potential to provide atomic level details to the surface dynamics controlling processes from surface chemistry to exotic collective effects. Accessing the primary timescales requires subpicosecond excitation pulses to prepare the corresponding nonequilibrium state. The needed excitation for maximum contrast above background invariably leads to photoinduced electron emission with the creation of surface fields that affect diffraction and must be quantified to recover the key structural dynamics. Using 2 keV ultrashort low-energy electron bunches, we investigate this field effect on the ensuing electron distribution in projection imaging and diffraction as a function of excitation intensity. Using a structural model, we demonstrate a quantitative separation of the surface field effect on electron diffraction, enabling isolation of the structural dynamics of interest. Particle trajectory simulations provide insight into the correlation between geometrical characteristics of the charge separated region and the corresponding intensity modulation at the detector.