position:absolute;的DIV 如何居中

外层DIV{position:realtive}
内层DIV{positon:absolute;top:50%;left:50%;margin-top:-100px;margin-left:-150px;width:300px;height:200px;}

<div class="father">
    <div class="son">
    </div>
</div>

.father{
    position:realtive;
    width:100%;
    height:100%;
}
.son{
    position: absolute;
    left: 50%;
    top: 50%;
    z-index: 1;
    width: 1200px;
    margin-left: -600px;
    height:600px;
    margin-top:-300px;
}