We study the effect of photodiode angular response on the measurement of surface plasmon resonance (SPR) in metallic thin films using the Kretschmann-Raether configuration. The photodiode signal depends not only on the light intensity but also on the incidence angle. This implies that the photodiode sensitivity changes along the SPR curve. Consequently, the measured SPR spectrum is distorted, thus affecting fits and numerical analyses of SPR curves. We analyze the magnitude of this change, determine when it is significant, and develop a calibration method of the experimental setup which corrects for this type of spectral shape distortions.