An optical image of an object is incident on a light-receiving unit of a two-dimensional matrix through a photographing lens. An output from the light-receiving unit is input to a first arithmetic logic unit to calculate an actual object brightness value in consideration of an aperture value of an aperture. An output from the first arithmetic logic unit is input to a multiplexer and a neural network. The neural network determines a main part of the object from brightness value pattern as a set of brightness values of the photoelectric transducer elements and outputs a position signal representing the main part. A multiplexer selectively passes only brightness values of the photoelectric transducer elements corresponding to the main part of the object from the outputs from the first arithmetic logic unit. An output from the multiplexer is supplied to a second arithmetic logic unit, and the second arithmetic logic unit calculates an APEX calculation on the basis of a speed value, an aperture value, a time value, and a mode signal representing a shutter or aperture priority operation, thereby determining a shutter speed or an f-number.