Explaining concretely: a uniform spatial displacement of the image corresponds to a ramping phase shift across the frequency spectrum. i.e. if you shift an ima

Explaining concretely: a uniform spatial displacement of the image corresponds to a ramping phase shift across the frequency spectrum. i.e. if you shift an image by 1cm, then the 1 rad/cm frequency component gets its phase "fast forwarded" by 1rad, the 1.5 rad/cm component forwarded by 1.5rad, and 2 rad/cm by 2rad and so on. By subtracting each frequency's phase from their original distribution, you are basically displacing them each by a different distance from one another, decohering the image entirely.