According to the properties of parabola, the distance from the point on the parabola to the sight line is equal to the distance from the focus to the sight line, so let the shooting points of A and B to the sight line be A', B', so AF = AA', BF = BB', and M is the midpoint of AB, so MM'= 1/2(AA'+BB'), so the problem is transformed into one. Knowing the size of the angle, the answer comes out, that is, the minimum when AF=BF, and then according to the simultaneous equation Y = x 2, the answer is 1.