An image synthesizing method includes capturing a first image using a reference small aperture size; capturing a second image using a reference large aperture size; obtaining one or more reference color weights according to a corresponding pixel of the first image and adjacent pixels of the corresponding pixel of the first image and a corresponding pixel of the second image; obtaining an associated distance by looking up an association table according to the one or more reference color weights; obtaining one or more associated color weights by looking up the association table according to the associated distance and an expected aperture; and obtaining a color value of a corresponding pixel of a synthesized image, by applying weighting to the corresponding pixel of the first image and the adjacent pixels of the corresponding pixel of the first image with the one or more associated color weights.