Patent ID: 7515201

Claim:
A focus detection apparatus for detecting a focus state of an object image in accordance with a contrast of said object image, said focus detection apparatus comprising: a lens drive mechanism for moving a focusing lens group of a photographing lens system within a predetermined range of movement; and a controller for detecting a contrast value of said object image at each of a plurality of different positions of said focusing lens group and for moving said focusing lens group, via said lens drive mechanism, in a direction from one end to the other end of opposite ends of said range of movement of said focusing lens group, wherein said controller calculates a difference between a maximum contrast value and a minimum contrast value of said contrast values detected at said different positions of said focusing lens group, wherein said controller compares contrast values at two adjacent positions of said focusing lens group, in said direction from said one end to said other end of said range of movement of said focusing lens group, to determine whether said contrast value increases successively a predetermined number of times and subsequently decreases successively a predetermined number of times, wherein said controller detects a maximum contrast value when said controller determines that said contrast value increases successively said predetermined number of times and subsequently decreases successively said predetermined number of times, wherein said controller calculates said maximum contrast value by calculating a point of intersection between straight lines of two linear approximation expressions when said difference is one of equal to or greater than a predetermined value, one of said two straight lines passing through points of at least two adjacent contrast values on one of opposite sides of said maximum contrast value, the other of said two straight lines passing through points of at least another two adjacent contrast values on the other of said opposite sides of said maximum contrast value, and wherein said controller calculates said maximum contrast value from a maximum value of a quadratic approximation expression which passes through a plurality of contrast values including said maximum contrast value when said difference is smaller than said predetermined value.