AUTOFOCUS SYSTEM

In an autofocus system in which the image definition of an object to be picked up is detected by image sensor means disposed on a light path of an image-forming optical system and converted into electric signals, and the image-forming position of the image-forming optical system is controlled by using the electric signals, a plurality of electric signals representative of image definitions are obtained from a main image sensor and at least one sub-image sensor which are provided at optically different positions on the light path and subjected to a predetermined logical judgement processing to produce a focus control signal. The position of the image-forming optical system is controlled by the focus control signal. When the sub-image sensor is movably supported and the light path length between the image-forming optical system and the sub-image sensor is varied by changing the position of the sub-image sensor, a signal representative of the position of the sub-image sensor is also subjected to a predetermined logical judgement processing and used for controlling the position of the sub-image sensor.