A distorted on-axis wavefront: the photon passes the pupil at point A at time t, while it crosses the pupil at point B at a later time t + dt. This induces different arrival times on the detector. The two events (the passage of the pupil at points A and B) are thus distinguishable, and the position of the photon in the pupil plane can be inferred with a precision better than D. This improved precision implies a larger uncertainty on the momentum: the images are blurred.