A measurement method is reported to distinguish the different mechanisms that lead to image lag in image sensors based on pinned photodiodes (PPDs). This new method can differentiate between the lag caused by a potential barrier or pocket and gate traps independently of charge spill-back. This is in contrast to usual lag characterization techniques, which only provide a quantitative measurement of the photocharge amount that is not transferred to the sense node in the frame of interest. The method reveals itself as useful to optimize pixels for high-speed and low-noise sensors based on PPDs.