It gets a little geeky, but by applying the principles of the Inverse Square Law, you’ll shoot better images.
The inverse square law describes how the intensity of light diminishes as the distance between the light source and the subject increases. This principle of the inverse square impacts lighting significantly. Understanding it helps us achieve the atmosphere and mood that we envision.
In this article, we answer questions such as: What is it? How does it work? Why is it important? And how do we apply and use it in photography?
Contents
What is the Inverse Square Law?
The Inverse Square Law originated with Sir Isaac Newton and the law of gravity. It forms one of the foundations of physics and applies to a number of physical phenomena, such as light, sound, and radio waves. Briefly, the law states that the illumination or brightness of a subject is inversely proportional to the square of its distance from the light source.
In practical terms, this means that if we double the distance from the light source to the subject, the intensity of that light reaching the subject is reduced to one-quarter of its source value.
The formula is below.
How the Inverse Square Law Works
When a light source moves away from a subject, the light falling on the subject diminishes. This is called light fall off and it follows the principle of the inverse square. In addition, the path of the light expands. And we’ll get into that in a moment.
In practical terms, if a photographer is taking a portrait and doubles the distance between the light source and the model, the difference in the light intensity reaching his subject is dramatically reduced.
This principle is crucial to photography, as it directly affects exposure, lighting setups, and the overall quality and mood of the photograph. In the photo below, six eggs are aligned and are six inches apart. The egg on the far right is closest to the light source and has some blown out highlights. The two eggs in the middle show more even lighting. At the far left, the egg is dimly, but more evenly lit.
Why this Principle is Important
The law is important because it’s a step toward mastering photography. That is, it allows for precise control over lighting conditions, not fumbling around with trial and error. In applying this knowledge, photographers manipulate light and dark, create specific moods, enhance textures, and achieve the desired tones in their images.
Additionally, this principle aids in the effective use of reflectors, diffusers, back lights, and other lighting modifiers, enabling photographers to shape and direct light more effectively. Ultimately, a solid grasp of the law enhances your technical proficiency. Also, it empowers photographers to express their artistic vision with greater clarity and impact.
The Dramatic Falloff of Light
The law tells us how quickly light falls off. That is, a subject turns darker sooner than you might expect when captured by a camera. That’s because the human eye adapts to different lighting conditions while the camera records what’s there.
The graph below illustrates the Inverse Square Law by the numbers. One foot from the light source is the starting point. If we double that to two feet, the fall off is 25 percent of the intensity at one foot. And at four feet, it’s 6.25 percent. At six feet, it drops to 2.78 percent, then 1.56 percent at eight feet, etc.
Of course, this influences both the exposure settings and the overall composition. For instance, when using a flash or continuous light source, a photographer must consider the distance to the subject. Make sure that the lighting is adequate and even, as subjects further away will receive significantly less light than subjects that are near. This potentially results in darker, underexposed areas in a photograph. Understanding this relationship allows photographers to manipulate light effectively, ensuring that their images are well-lit and visually appealing.
How to Use the Inverse Square Law in Photography
This principle directly impacts exposure settings, lighting arrangements, and the overall mood. By precise control of the light source distance, photographers create the effect they’re looking for. As an example, you might increase the intensity of the light by moving the light source closer to the subject and a little to the side.
This creates dramatic shadows, as shown in the photo on the left below. Meanwhile, the photo on the right shows the result of moving the light source away from the subject. This tends to soften those shadows and create a more even skin tone.
In both cases, notice that the Inverse Square Law is in effect. As a photographer, you probably won’t calculate the exact distance and change in light intensity. However, you notice how small changes in the light source affect the subject.
The Relationship Between Light Source and Aperture
To review, the Inverse Square Law states that when the distance between the light source and the subject doubles, the light intensity decreases to one-quarter. In addition, as light travels across a distance, it expands outward. So, the light falls off, and less of the original concentrated light power illuminates a wider and higher area.
Meanwhile, the aperture setting changes. Let’s say you’re shooting a portrait and the light source is one foot away and your aperture setting is f/11. Then, if you move the light source to two feet away, you reduce the light intensity to one-quarter and the aperture setting in this case would be f/5.6. And again, if you move the light source to four feet away, you see an additional light fall off to 1/16. And the aperture setting would be f/2.8.
A light meter will do the math in these cases. However, a working knowledge of the principle and its application gives the professional photographer an advantage. You not only enhance the technical quality of your images, but you also know how to manipulate light and shadow to convey mood and capture the atmosphere that you’re trying to create.
Overcoming Common Mistakes in Lighting
We encounter several common pitfalls when it comes to lighting. One frequent mistake is failing to account for the distance between the light source and different parts of the subject, leading to uneven lighting. For example, a portrait may fall victim to light fall off. If the light source is close to the subject, her forehead may be slightly overexposed. This results in uneven tones, harsh shadows, and unflattering highlights. This can be remedied by moving the light source away from the subject and adjusting the aperture to compensate.
Similarly, let’s say we’re photographing a group shot of three rows of people. Then in post processing, we discover that in the second row the faces are slightly underexposed and in the third row even more so. Understanding how quickly light falls off, we compensate for this fall off by moving the light sources further away. Also, raising the light source adds balance to the three rows, giving us more even exposure across the group.
Understanding and applying the principles of the inverse square law significantly enhances a photographer’s skill set. It gives him or her the ability to create a properly exposed, balanced, and visually appealing image.
Compensating for Differences in Skin Tone and Hair Color
The Inverse Square Law helps us equalize exposure for different skin tones. If you’re shooting two people with contrasting skin color, place the darker toned person closer to the main light source. And position one light on the subject with the lighter tone a little further away.
Contrasting hair color presents another challenge. For example, blonde hair that’s brighter than the subject’s face or black hair that’s darker than the face means a slight difference in exposure. In both cases, moving the light farther away from the subject helps to balance the exposure. Remember, as the light moves further away, we minimize the difference between light hitting the hair and light falling on the face.
The 90 Percent Rule of Lighting
Another fundamental principle of photography, this applies to reflected and absorbed light. And understanding this rule helps in application of the Inverse Square Law. Pure black absorbs 90 percent of the light striking it. Pure white reflects 90 percent of the light that strikes it.
Of course, not all black and white shades are pure. But quite a few are close. So, remember the 90-percent rule. And learn to evaluate shades of gray and color tones that fall between black and white.
How the Inverse Square Law Impacts the Background
When working in a studio, you can change the brightness level of the background by its proximity to the subject. If you place the subject twice the distance from the background as from the light, the light intensity on the background or wall is one-fourth that of the subject. This ensures that the background will be darker than the subject, making the subject stand out.
As another example, you may opt for a lighter background, again utilizing the effect of the law. If the distance between the subject and the light source is twice the distance between the subject and the background, the background will appear brighter than in the previous example.
The series of shots below shows how background illumination changes as the relationship between the subject and background changes.
Photo 1: the subject is two feet from the background and six feet from the light source.
Photo 2: the subject is four feet from the background and four feet from the light source.
Photo 3: the subject is six feet from the background and two feet from the light source.
In all three photos, the light source remained the same, and exposure was set for the doll’s face. The series demonstrates how we make the background brighter or darker using the Inverse Square Law.
Conclusion
The inverse square law, a fundamental principle in physics, describes how light diminishes as the distance from the light source increases. To review, the illumination or brightness of a subject is inversely proportional to the square of its distance from the source of that light.
This law helps photographers understand how to adjust lighting to achieve the desired effects in their shots. Remember that the distance between the light source and the subject also affects exposure, depth of field, and the overall mood of the photograph. Additionally, photographers should keep in mind that adjusting the distance can lead to dramatic changes in shadows and highlights. And a change in light means changes in camera settings.
I hope this article clarifies a complex issue in photography. If you have any questions or comments, please submit them in the space below.
Take Away
The Inverse Square Law states that the illumination on a subject decreases proportionally to the square of the distance from the light source. This means that if you double the distance, the light intensity becomes one-fourth as strong. Understanding this concept is crucial for photographers, as it influences exposure settings, lighting arrangements, and the overall quality and mood of images captured.
