Method of automatically focusing using a quadratic function in camera

A method of automatically focusing a camera determines a position of a focus lens where an amount of a high frequency in an image signal is a maximum. The amount of a high frequency included in an image signal is measured at intervals of multiple focus motor steps. Three of the measured positions (x1, x2, x3) and their corresponding measured high frequency amounts (y1, y2, y3) are identified, where y2 is the largest of the measured high frequency amounts, and x1 and x3 are the nearest measured positions on either side of x2. A quadratic function f(x) is determined for the high frequency amount with respect to the focus lens position x. A position at which a derivative 1 f ( x ) x of the quadratic function f(x) is zero, is determined to be the position corresponding to maximum high frequency amount. Thus, the driving time of the focus motor can be reduced.