Stops of Light

So what is a Stop? 

In this example of Martin (The Bridge - Broen) the key light si reading 3 stops brighter than the fill light, of course this could be certain if measured with a meter on a set. If wanted to reproduced this shot or a similar light/dark feel to it, we would need 3 times the amount of light on the key side compared to to the fill side. 

Getting to shoot a scene like this one need to get the key light read F2.8  and setting up the (ISO) rating the camera and the shutter speed you are focusing on. In this shoot the key background and it measures of that of a whole.  The key to background ratios setting up the mood for the viewer and helps a cinematographer to shape where he or she wants the audience to focus their attention in a given frame.