1. An automatic focus searching method for an image capture device, the image capture device having a lens and an image sensor having a rolling shutter readout mechanism, the method comprising: setting an exposure time of the image sensor; computing a focal sweep trajectory of the lens based on at least the exposure time; during the exposure time, moving the lens along the focal sweep trajectory while exposing the image sensor; reading out pixel data of the image sensor that is generated during the exposure time; and applying a filtering operation upon the pixel data to generate an output image; wherein the step of computing the focal sweep trajectory of the lens comprises: providing a plurality of regions of interest (ROIs) corresponding to the output image; determining at least one target ROI from the ROIs according to a user input selection; and determining an optimal lens position corresponding to each target ROI; and computing the focal sweep trajectory according to an exposure sequence corresponding to each target ROI and the optimal lens position corresponding to each target ROI; wherein the plurality of ROIs include a first target ROI and a second target ROI, a first exposure sequence corresponding to the first target ROI is prior to a second exposure sequence corresponding to the second target ROI, and the step of computing the focal sweep trajectory of the lens comprises: setting the focal sweep trajectory of the lens to have a start position that is close to a first optimal lens position corresponding to the first target ROI; and an end position that is close to the second optimal lens position corresponding to the second target ROI; and setting the start position to correspond to a start time of the exposure time and the end position to correspond to an end time of the exposure time wherein first pixels of the image sensor corresponding to the first ROI are exposed before second pixels of the image sensor corresponding to the second ROI are exposed.