In　previous　literature,　most　researches　about　auto-focusing　technique　for　the　whole　image　concentrate　on　the　image　definition　function　for　all　the　pixels　of　an　image　which　is　time-consuming.　This　paper　presents　an　improved　auto-focusing　method　based　on　regional　contrast　and　down-sampling　after　analyzing　the　different　image　definition　functions.　Experimental　results　prove　that　the　downsampling　scheme　is　feasible,　universal,　and　time-saving,　and　the　proposed　method　outperforms　the　other　method.　©　2014　WIT　Press.