Darken image overlay and add text over it in CSS

Question

How would I darken (add a semi-transparent overlay) and add text to this image (but centred horizontally and vertically) as below:

Answer 1

html

fsfsfsssf

css

body{
    margin:0px;
}
#back{
    width:100%;
    height:500px;
    background: url("http://luxurylaunches.com/wp-content/uploads/2014/05/uber-london.jpg") no-repeat;
    background-size:contain;
}
#mask{
    position:relative;  
    width:100%; 
    height:500px; 
    background:rgba(255,255,255,0.5); 
}
#text{
    position:absolute;
    top:230px;
    left:48%;
}

Fiddle: http://jsfiddle.net/6jf0nxd5/20/