>>2593746that pretty much depends on the speed of the subject, the angle it is traveling relative to you, and the distance the subject is from you
the slower you go with shutter speed, the smaller the sharp area will be (unless the subject is travelling perfectly perpendicular to your line of sight)
try to picture the motion blur when watching the subject through your viewfinder, and panning with it
whenever part of the subject moves in the frame, that will come up as blur if you were to take a photo at that time
this shot was taken with a slow shutter speed, and with the subject traveling at an angle relative to the lens rather far from 90°, so the sharp area is rather small
