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Learn the Inverse Square Law in Photography: How Light Behaves and Affects Your Images

This article is a technical introduction to lighting for photographers who want to learn more about the science of photography.

Defining the Inverse Square Law

An image depicitng the science behind the inverse square law. it shows the math and distribution from a singular "point of light"
The Inverse Square Law

Learning the inverse square law is an essential principle in photography that relates to how light behaves and affects the images we capture. It profoundly impacts how we light and expose our subjects and is something every photographer should understand to produce the best results.

The inverse square law states that the amount of light that falls on a subject is inversely proportional to the square of the distance from the light source. Therefore, as the distance between the light source and the subject increases, the light that falls on the subject decreases rapidly.

For example, a light source 1 meter away from your subject will provide a certain amount of light. However, if you move that light source 2 meters away from your subject, the amount of light will decrease by a factor of 4. And if you move it 3 meters away, the amount of light will reduce by a factor of 9.

The Inverse Square Law in Portrait Photography

In portrait photography, light plays a crucial role in creating the desired effect, and the distance between the light source and the subject is a critical factor in determining the overall look and feel of the image. As the light source moves farther away from the subject, the amount of light that falls on the subject decreases, and the light becomes more directional. This means the shadows will become more prominent, resulting in a more dramatic, moody look.

A black and white image of a young woman with elegant earrings looks over her shoulder. Part of her is obscured by darkness while a rim light separates her from a dark background.d
a softer, flattering look with fewer harsh shadows

Conversely, moving the light source closer to the subject increases the amount of light that falls on the subject, and the light becomes more diffused. This can result in a softer, flattering look with fewer harsh shadows.

The size of the light source also affects the portrait's overall look, with larger light sources providing softer, more diffused light and smaller light sources providing more directional, harder light.

This is because bigger light sources create more even illumination by bouncing light off a broader area, whereas smaller light sources produce more directional light.

A young woman is mostly obscured in darkness. She's wearing a cowboy hat while her cheek, nose, lips, chin, hand, and hair a properly exposed. A dramatic portrait.
a more dramatic, moody look

For instance, a small light source, like a spotlight, produces a hard light with pronounced shadows, while a significant light source, like a softbox or umbrella, creates soft, even illumination with little or no clouds.

The inverse square law has important implications for photographers because it affects how we light our subjects and expose our images.

For example, if you want to light a portrait and maintain a consistent brightness level as the subject moves away from the light, you will need to increase the power of the light source.

Similarly, suppose you're photographing a natural landscape blending natural and artificial light and want consistent exposure across the scene. In that case, you must consider the inverse square law and adjust your equipment settings accordingly.


Another way to think about The Inverse Square Law is in terms of Light fall-off.

Light Fall Off

Light fall-off is a phenomenon where light intensity decreases as it moves away from the light source. In photography, this means that the light that falls on a subject is brightest where it's closest to the light source and gets progressively dimmer as it moves away from the source.

This has significant implications for the contrast of the subject in the photograph. As the light's intensity on the subject decreases, the shadows become darker, increasing the contrast between the light and dark areas of the image. The result is that the subject can appear more three-dimensional, with greater depth and texture, as the contrast between Light and shadow increases.

Photographers can use this effect to create various visual effects in their images. For example, if the photographer wants to make a portrait with a dramatic, high-contrast look, they may position the light source close to the subject to produce a rapid fall-off and a sharp transition between the light and dark areas of the image.

On the other hand, if the photographer wants a softer, more even look, they may use a larger light source positioned farther away from the subject. A larger light source will produce a more gradual fall-off, with a gentler transition between the light and shadow areas, resulting in a softer and more diffused appearance.

Understanding light fall-off and its effects on contrast is an essential aspect of lighting for photography. It enables photographers to create various visual effects, from high-contrast drama to soft, even illumination. By mastering this aspect of lighting, photographers can achieve greater creative control over the look and feel of their images.

Learning The Inverse Square Law in Practice

The inverse square law can be a complex principle, but it is essential for creating the best images possible. So, whether you are shooting portraits, landscapes, or anything in between, understanding the inverse square law and how it affects your pictures will help you to produce stunning results every time.


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