Disclosed is an image sharpness detecting system in which an image formed by an image forming optical system is received by an array of a plurality of optical-electronic transducer elements, and an illumination difference in each individual minute compartment of the above-described image is detected on the basis of the concurrently obtained output signal from a corresponding one of the optical-electronic transducer elements, while all the illumination differences are summed up over the entire area of a sensing region of the above-described image, whereby is obtained a signal representing the degree of sharpness of the image on the above-described array of optical-electronic transducer elements. In this system, the signal representative of the illumination difference in each individual minute compartment of the above-described image is at least non-linearly transformed by utilizing the non-linear characteristics for output versus input of a semiconductor element such as a transistor, and the signal representative of the degree of sharpness of the above-described image is obtained on the basis of the non-linearly transformed signal.