A method of comparing analog signals delivered from a number of light detecting elements with analog signals delivered from a digital-analog converter by means of a plurality of comparator circuits whose output analog signals are supplied as write enable signals to a plurality of digital memories which are controlled by means of digital signals delivered from a central control circuit such that an illumination intensity information of each picture element is read out as the digital signal in the digital memories, thereby obtaining a signal of detecting a focal point of cameras or the like optical devices.