Difference between optical flow field and motion field?
well, have you ever seen a rotating barber's poll?

It looks like the stripes are moving up (optical flow), but of course the motion of the thing is a rotation in the horizontal plane.



The rationale really is that the optical flow is based on how the image changes. And if you just compared two frames from the animation above, you'd clearly say that the blue stripe moved upwards. However, if you'd open the device and looked inside, you'd see the motor that causes the mechanical movement (rotation).