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 by at least: determining a position of a center of a desired field of view for exposure on the image in response to the selection of the region of interest or the point of interest; and determining the target position of the X-ray source based on the position of the center of the desired field of view for exposure on the image and imaging parameters of the image sensor by at least: determining an actual position of the center of the desired field of view for exposure on the patient based on the position of the center of the desired field of view for exposure on the image and the imaging parameters of the image sensor; and determining the target position of the X-ray source based on the actual position of the center of the desired field of view for exposure on the patient and inherent positional relationship between the X-ray source and the center, wherein the 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.