They represent difference between highlights and the shadows in a scene. It's amazing how many images are being made flat these days. Understanding simple ratios tells you a lot about your scene. It helps you control depth and shape and shadow. It helps you visualize and make an image work your way.
Lets imagine a face in a portrait, whether strobe lit or natural lit. Lets say you meter and light on the shadow side reads 1 stop darker than it does on the highlight side (you can even measure this with your in camera spot meter). This means double the light is reaching the highlight side, or a 2:1 ratio.
Anytime you increase or decrease by a stop, you double your light and thus your ratio.
A few highlight/shadow ratio examples…
1 stop diff = 2:1
1½ stop diff = 3:1
2 stop diff = 4:1
3 stop diff = 8:1
Now I certainly don't claim to be the grand master of ratios, I'm studying them further all the time. But at their core they are another simple powerful way for us to better work with and understand our light. We'll be looking at them further during my EXposed series... http://seimeffects.com/exposed
There's certainly more to discuss on this topic. Lets do it...