An imaging system is provided that includes a target detector, a readout area determiner and a readout processor. The target detector detects a target subject from an effective pixel area of an image sensor. The readout area determiner defines a readout area within the effective pixel area, the readout area corresponding to a detected target. The readout processor reads out only pixel signals within the readout area. A partial area within the readout area is redefined as the readout area when the size of the original readout area is greater than a predetermined size.