1. An auto focus method comprising: (a) obtaining a selected exposure value and an un-normalized contrast value, wherein the un-normalized contrast value corresponds to a stepping position; (b) executing a normalized contrast value obtaining procedure, wherein the normalized contrast value obtaining procedure comprises: (b1) obtaining a next un-normalized contrast value and a next exposure value of a next image at a next stepping position; (b2) determining if the next exposure value and the selected exposure value satisfy a renew condition, and renewing the selected exposure value when the next exposure value and the selected exposure value satisfy the renew condition; and (b3) obtaining a normalized contrast value corresponding to the un-normalized contrast value and a next normalized contrast value corresponding to the next un-normalized contrast value according to the renewed selected exposure value; and (c) executing a focal point searching procedure according to the normalized contrast value and the next normalized contrast value, wherein the focal point searching procedure comprises: (c1) determining if a reverse condition is satisfied according to the normalized contrast values; (c2) outputting the stepping position corresponding to the maximum of the normalized contrast values if the reverse condition is satisfied; and (c3) executing the normalized contrast value obtaining procedure if the reverse condition is not satisfied.