Today, I’m excited to kick off a new series where we break down and define common photography terms that you often hear but might not fully understand. This series aims to help you grasp these concepts and use them effectively in your photography. Let’s dive into our first topic: The inverse square law.
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Understanding the Inverse Square Law
Understanding the inverse square law can significantly improve your lighting techniques and overall image quality. The inverse square law states that the intensity of light is inversely proportional to the square of the distance from the light source. In simpler terms, as you move your light source further away from your subject, the light intensity decreases dramatically.
For example, if you double the distance of your light from your subject, the light’s intensity doesn’t just halve; it actually reduces to a quarter of its original intensity, which is two stops darker. The easy to remember rule of thumb is “double the distance, quarter the light.” This rapid fall-off of light is crucial to understand when setting up your lighting, whether you’re in a studio or on location.
Controlling Light Fall-off
The inverse square law helps you control how quickly the light falls off from your subject to the background. If you want a dramatic portrait with a well-lit subject and a dark background, place your light source close to your subject. The light will fall off quickly, leaving the background in relative darkness.
If the background is still too bright, move your subject and light further away from the background. On the other hand, if you want a bright background and it’s too dark, consider moving your subject and your light closer to the backdrop. In some instances, you may need to light the background with more lights. I’ll link to a video that is all about lighting backdrops.
Problem Solving
If you ever find yourself in a scenario where your light at minimum power is too bright, just move it backwards. If you ever find that it’s not bright enough at maximum power, try moving it closer, which not only increases light intensity but also softens the light, because the larger the light source looks to the subject the softer it will be.
Demonstrating the Inverse Square Law
To illustrate the inverse square law, let’s do a quick demonstration. First, we’ll place a light with a bare bulb perpendicular to our backdrop and photograph it to show the rate of fall-off. This will give you a clear visual of how light intensity decreases with distance. Every time we double the distance from the light we lose 3/4 of the light. The key takeaway here is it falls off fast and as you get further away there is less of a difference in brightness.
We’ll start with the light source 1 meter away from the subject. Notice how the subject is well-lit, and the background falls into darkness quickly. The light intensity is at its maximum here.
Now, let’s move the light source to 2 meters away. It’s going to be 1/4 as bright so we will increase our ISO two stops to compensate. Observe how the light on the background starts to become brighter. This happens because there is less of a difference between the distance from the light to the subject and the distance from the light to the background.
Finally, we’ll move the light source to 4 meters away and once again increase our ISO two stops to compensate for your light becoming dimmer on the subject. At this point, the light on the subject and the background are almost the same.
Close-Up Portrait Fall-off
Now, let’s demonstrate how the inverse square law affects a close-up portrait. We’ll start with the light source very close to the subject’s face. Notice how the light falls off quickly, creating a dramatic hot spot that falls off quickly.
Next, we’ll move the light source further away from the subject. Observe how the light becomes more even across the face.
Please take a moment to repeat my experiments with different light-to-subject distances so that you can see how the light fall-off changes and how it affects your images. This hands-on practice will help you internalize the concept.
Conclusion
Understanding and applying the inverse square law can dramatically improve your lighting setups and overall photography. By mastering this principle, you’ll have greater control over light fall-off, intensity, and the mood of your images.
If you found this video helpful, please like, subscribe, and sign up for the bell. Consider sending a Super Thanks below to support me to make more videos like this one. Also, please suggest future topics for this series in the comments and check out my online members-only learning platform, the Academy with John Gress. Thank you so much for watching. Have a fantastic day, and I’ll talk to you soon.
Chapters
00:00 – Introduction
00:31 – Understanding the Inverse Square Law
01:27 – Controlling Light Fall-off
02:10 – Problem Solving
02:38 – Begin Demo
04:06 – Closeup Demonstration
04:28 – Homework
04:46 – Conclusion
