I have been playing around with getting good panning shots. Going out to the go-kart track to practice. It seems that on a lot of my photo the subject comes out segmentally focused. Where only the front, or back or top or bottom of the subject is sharp and the rest gets caught in the motion blur. I don't understand why? the front of the kart is moving the same speed as the rear, if I got the front focused (panned at the same rate it was moving) then shouldn't the rear be sharp too? I am shooting a telephoto lens. Any ideas?