An auto focus method includes: assigning searching boundaries L=InitL, R=InitR, and acquiring the resolutions f(L), f(R); defining the focused positions X1, X2 by the golden section method, and acquiring the resolution f(X1), f(X2); determining whether or not f(X1) is greater than f(X2); moving L adjacent to R when f(x1) is less than f(x2); moving R adjacent to L when f(x1) is greater than f(x2); acquiring the focused positions L, x1, x2, and R, and the resolution of the focused positions when L≠InitL and R≠InitR; acquiring an optimal focus point according to the focused positions L, X1, X2, and R, and the resolution of the focused positions.