1. An image capturing apparatus, comprising: a main image capturer performing an image capturing operation on a target object according to a detection focusing distance; an auxiliary image capturer disposed adjacent to the main image capturer, and the auxiliary image capturer performing a plurality of focusing operations on the target object according to a plurality of focusing distances, and generating a plurality of image contrast values; and a controller coupled to the main image capturer and the auxiliary image capturer to control the main image capturer and the auxiliary image capturer, and the controller selecting one of the focusing distances to be the detection focusing distance according to the image contrast values.