1. An automatic focusing method for an optical system, comprising: performing an initial coarse focus action along a focal axis at a scan position corresponding to a point on a surface of a slide; and respectively performing a plurality of subsequent fine focus actions along a plurality of focal axes at a plurality of scan positions corresponding to different points on the slide surface, performing the plurality of subsequent fine focus actions comprising obtaining a plurality of images of the slide at a plurality of coordinates, each image comprising a two-dimensional array of pixels, wherein each of the fine focus actions is performed more quickly than the initial coarse focus action.