A Simple Guide to the Inverse Square Law

1/01/2025 ISO 1200 Magazine 0 Comments

Without a solid grasp of fundamental lighting principles, it’s challenging to produce consistent and artistic results. This video will guide you through the basics of the Inverse Square Law, a key concept that helps photographers understand how light behaves in relation to distance.

What is the Inverse Square Law?

The Inverse Square Law is a physics principle that applies directly to light. It states:

The intensity of light equals one over the square of the distance from the source.

Mathematically:

I \propto \frac{1}{d^2}

Where:

$I$ is the intensity of the light.
$d$ is the distance from the source.

This means that if you double the distance from the light source, the intensity becomes one-fourth of the original value. Similarly, if you triple the distance, the intensity becomes one-ninth.

In simpler terms, as the distance from a light source increases, the intensity of the light decreases dramatically. For example:

At 1 foot, light retains 100% intensity.
At 2 feet, the light's intensity drops to 25% (1/2²).
At 3 feet, the intensity becomes approximately 11% (1/3²).

This demonstrates how quickly light “falls off” (loses intensity) as it travels further from the source.

Why Is It Important for Photographers?

Understanding the Inverse Square Law allows photographers to:

Control Lighting on Subjects: By placing the light source closer to the subject, the light appears more intense and falls off quickly, creating strong contrasts between the subject and the background.
Achieve Balanced Lighting: By positioning the light farther away, the intensity spreads evenly, making it ideal for group shots or evenly lit scenes.

Practical Applications in Photography

Portraits:
For individual or close-up portraits, a light source placed closer to the subject helps create a dramatic effect with more emphasis on the face. Shadows fall off rapidly, creating depth and contrast.

Group Photos:
For group photography, positioning the light farther away ensures even coverage. This minimizes differences in brightness across the group and lights the background effectively.

Creative Photography:
Understanding light fall-off allows photographers to create artistic effects by strategically lighting specific parts of a subject while leaving others in shadow.

Demonstration of the Inverse Square Law

Ben Marcum provides a demonstration of how the Inverse Square Law works in practice.

Light Placement and Fall-off: When the light source is placed closer to the subject, the background remains darker due to rapid fall-off.

Effects of Distance: As the light moves farther back, both the subject and the background receive more even lighting, and the light's intensity decreases, requiring an adjustment in power.

For example, in the video, a 4-inch Octabox is used to illuminate the model’s face. When the light is moved closer, her face is brighter and the background darker. Moving the light back brightens both the background and her body more evenly.

Mastering the Inverse Square Law empowers photographers to manipulate light creatively, ensuring that every shot aligns with their artistic vision.

Images and video via Ben Marcum Photography