A method of detecting focusing conditions used for optical machinery is disclosed. The focusing conditions of an optical image projected onto photodetector arrays arranged on both sides of the focal plane with a certain optical distance are detected based on an evaluation function representing a sum of absolute values of differences between output signals of adjacent photodetectors. When a value of a first evaluation function represented by the largest absolute value is less than a predetermined threshold a second evaluation function represented by a sum of the maximum and next largest absolute values is used thereby detecting the focusing conditions for a graded image.