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;
}