A practical real-time auto-focus algorithm for a digital camera is presented, and it improves the reliability and speed of the auto-focus process, especially suitable for a mega-pixel high definition camera. The proposed algorithm adopts threshold gradient and edge point count technique besides focus value function, instead of the traditional two-stage climbing search algorithm that uses the focus value function only. Additionally, a relative difference ratio circuit is also proposed, which can implement adaptive step size searching to increase the searching speed. By adopting the modified algorithm on the prototype of our mega-pixel digital camera, the real-time auto-focus function is verified. The proposed algorithm is implemented in a test camera chip that has been manufactured in 0.25/spl mu/m CMOS digital process.