Provided is an optical device for reducing the time for auto focusing (AF) performed by a contrast detection system, the optical device comprises a liquid lens having a curvature that varies on the basis of an applied electrical signal and the liquid lens may be auto-focused at one time from the current curvature (i.e., the first curvature state, p) to a target curvature corresponding to a distance to the subject (i.e., a second curvature state) based on the FV slope ratio.