1. A method for automatically adjusting a field of view for exposure of an X-ray system, the method comprising: capturing an image of a patient on an examining table of the X-ray system by an image sensor, wherein the image sensor is placed at a predetermined position in the X-ray system; displaying the captured image on a display for selection of a region of interest or a point of interest by a user on the image; automatically determining a target position of an X-ray source in response to the selection of the region of interest or the point of interest on the image, wherein a desired field of view for exposure covering the region of interest or the point of interest is obtained when the X-ray source is located at the target position; and automatically locating the X-ray source at the target position in response to the determination of the target position.