In this video and article I’ll explain the inverse square law of light and how it is one of the most powerful tools in your arsenal.
First, watch the video:
What is the Inverse Square Law of Light?
Simply put, light loses energy as it travels. The further you move from a light source, the dimmer it appears. This is why there’s an ‘Inverse’ in the name.
But how exactly does it lose its intensity? Intuitively, you expect light to drop off (“fall off” is the commonly used term) linearly. Move twice as far, and it should be half as bright, right? Nope.
Light intensity decreases proportionally to the square of the distance. Move twice as far, and the light falls off by four times. Move ten times further, and it drops by a hundred times. That’s why there’s a ‘Square’ in the name.
Fall off is proportional to square of the distance between two measurement points.
If you measure light intensity at one spot and then at another further away, the distance between them dictates how much the intensity decreases. Don’t worry, you don’t have to do any math on set, but understanding this law helps immensely on set.
Why is it absolutely critical you know and use it?
If you’re ever going to light a film you can’t escape the inverse square law. It’s a critical and very useful tool for the cinematographer.
The closer the light source to the subject, the faster the fall off. The further it is away, the slower the fall off. That’s why, even though the sun is millions of miles away, it lights the land evenly.
Ever stand next to a window, one cheek facing out, the other inside? Assuming the sun isn’t directly hitting you, the shadow side of your face is much darker than the lit side.
That’s a fast fall-off, or a dramatic fall-off.
The source isn’t the sun, it’s the window, and it’s right next to your face. That’s why the fall-off is faster.
Now imagine the house vanishes, and you stand exactly as before, but now the sun hits you directly in the same angle as the window.
You won’t see the same fall-off. The shadow side will be much brighter, even if you eliminate all the reflections from the sunlight bouncing around and hitting you from all angles.
That’s because the sun is further away, the fall-off is slower.
When lighting locations, especially large sets, it’s important to keep the inverse square law in mind or you’ll just screw up. Maintain uniform lighting by carefully placing sources at calculated distances.
On the other hand, you can manipulate the mood by using rapid fall-off for suspenseful scenes or a gradual fall-off for romantic settings.
Understanding fall-off also helps you with balancing the key light and fill light (the lighting ratio) to make the scene appear natural. To understand how lighting ratios change moods in filmmaking, watch this video:
Once you get the hang of using the inverse square law, it’ll be second nature. Practice makes perfect.
Easy way to remember it
The easiest way to remember it is to forget ‘square’ and ‘double’ and what not. Cinematographers light in terms of stops. A change of one stop is double (or half) the light.
That means, every time you double the distance, you lose two stops.
Double the distance = Lose two stops
I hope this video and article has helped you understand the inverse square law. Keep thinking about it until it becomes second nature!
I am learning to be a Cinemetography it’s taking a minute to grasp the lighting concept.. but you video are so informative it’s forcing me to think outside box .. I just want to personally thank for your great videos . They have change me life ..
You’re welcome!