스크롤 내리면서 opacity 변경

<div id="main">
                    <div class="pc-view" id="bg"></div>
        </div>

 

 

$(window).ready(function(){
    var max = 500; //100% 투명할때의 스크롤 값
    $(window).scroll(function(){
        var scrollPX = $(this).scrollTop();
        if( scrollPX  < max ) {
            $("#bg").css({"opacity": (max-scrollPX)/max });
        }else{
            $("#bg").css({"opacity": 0});
        }    
    });
}); 
</script>