a focus detecting device of the kind using an area sensor which has a plurality of focus detecting areas necessitates setting a correction value for every one of the focus detecting areas and storing all the correction values thus obtained , whereas , according to the invention , the correction value for each of the focus detecting areas of the area sensor is obtained by an arithmetic operation using , as variables , two parameters indicative of the position of the area , so that a focus detecting device can be arranged to obviate the necessity of storing all the correction values for the respective focus detecting areas .