como fazer elementos que scroll diferente

$(window).scroll(function(e){
  parallax();
});
function parallax(){
  var scrolled = $(window).scrollTop();
  $('.bg').css('top',-(scrolled*0.2)+'px');
}.bg {
  background: url('http://i.imgur.com/USFbNpC.jpg') repeat;
  position: fixed;
  width: 100%;
  height: 300%;
  top:0;
  left:0;
  z-index: -1;
}
section {
  color: #fff;
  font-family: arial;
  width: 500px;
  margin: auto;
  line-height: 20px;
  font-size: 16px;
}