Temporal resolution in fast photonic tubes, e.g., streak tubes, frame tubes, and photodiodes, is enhanced by using a high electric field at the photocathode. However, at relatively high irradiance, the tube performance is degraded in both temporal and spatial resolution. It is generally assumed that the degradation is due to space charge. It is well known that space charge depresses the electric field in the immediate region of the cathode. For zero initial‐energy electrons, the electric field at the photocathode decreases with increasing space charge and becomes zero when the full space‐charge‐limit current is extracted from the cathode; however, photoelectrons from real cathodes have finite energy distributions that can change the polarity of the cathode field from accelerating to retarding at high irradiance. By utilizing appropriate photoelectron energy distribution functions, we will first calculate the electric field at the cathode‐grid region for high irradiance and then examine the perturbing effects upon the resolution in the photonic tube.