This paper models focus value curve from point light source to focus measure calculation. This model shows that focus values of one object in a scene will form a single peak bell-shaped curve in camera. Our proposed auto focus method adopts this model with focus sub-window selection to predict the peak of focus value curve. Accurate prediction for the best-focused lens position will greatly shorten search time. Besides, the period of focus value feedback from camera chip is further considered to determine lens step in one period to optimize practical time consumption. Through experiments, our model for focus value curve is correct and proposed auto focus method efficiently shortens overall search time.