Accurate auto-focus method

An auto-focus method configured for controlling a focusing lens of a lens module to move to an optimal focused position Xp, comprising:

• assigning searching boundaries InitL, InitR of the focusing lens to L, R respectively;
• acquiring the resolutions f(L), f(R) of images captured by the lens module when the focusing lens is positioned on the two searching boundaries InitL, InitR, wherein f(X) represents the resolution of the image captured by the lens module when the focusing lens is positioned on a focused position X, X is a variable value greater than InitL and lower than InitR;
• defining two focused positions X1, X2, wherein X1=L+(1−τ)×(R−L), X2=L+τ×(R−L), and τ=0.618;
• acquiring the resolutions f(X1), f(X2);
• comparing f(X1) with f(X2) to determine whether or not f(X1) is greater than f(X2);
• determining that L=X1, X1=X2, and X2=L+τ×(R−L) when f(X1) is lower than f(X2);
• determining that R=X2, X2=X1, and X1=L+(1−τ)×(R−L) when f(X1) is greater than f(X2);
• determining whether or not L≠InitL and R≠InitR are established;
• acquiring the focused positions L, X1, X2, and R and the resolutions f(L), f(X1), f(X2), and f(R) when L≠InitL and R≠InitR are established;
• and
• calculating the optimal focused position Xp according to the focused positions L, X1, X2, and R and the resolutions f(L), f(X1), f(X2), and f(R).