Light Intensity

020_JemezLightShadowIt is the change in light intensity that gives the night its dramatic edge.

The intensity of light from any source is not fixed. It diminishes as it travels outward from its origin.

In the daytime, sunlight is spread evenly across the surface of the earth, because the sun is essentially the same distance from every point on our planet.

As a result, we do not witness a change in light intensity at any given moment.

Not so at night. Once the sun sets, we are dealing with lighting from sources at much shorter distances than our solar neighbor.

The intensity of light falling on our subject is determined by the distance from its origin. So changes in light intensity from nearby light sources are fairly obvious, especially from single isolated light sources.

There is a simple explanation for this phenomenon, namely the Inverse Square Law, one of the fundamental laws of physics that pertains to light.

OK, that can sound a bit ominous. But in practice the concept is pretty straightforward. Mathematically it describes how light intensity decreases exponentially as the distance from a light source increases.

EQUATION : Inverse Square Law

∆ (light intensity) = 1 / ∆ (distance) ^ 2

The Delta symbol ∆ stands for change.

In words, it is the inverse of the change in distance squared that determines the change in light intensity.

The equation is not an absolute measure of intensity. Instead it describes how a change in distance effects a change in light intensity.

For example, the light intensity at 10 feet from a light source is 4 times as strong as it is at 20 feet. Put another way, the intensity is 1/4 as strong at twice the distance. Likewise, the intensity at 30 feet, three times the distance, is 1/9 as strong.

“So why do I care?”, you might ask.

The Inverse Square Law has several implications and applications in night photography.

We will take advantage of this law when we explore alternative lighting techniques in upcoming posts. For example, we will learn how to calibrate a flash or flashlight for supplemental lighting purposes.

As photographers, we are naturally drawn to uncommon lighting. Even though variations in nocturnal lighting can seem obvious, it is good practice to consciously seek out varying light intensities in our quest for more evocative image making.

IMAGE : Light & Shadow, Jemez Springs, NM

I hid the light source behind the tree branch to showcase the dancing trees, the spreading shadows, and the gradation of light on the snow without distraction.

I also wanted to avoid any lens flare or aperture starring that might draw attention away from the more subtle elements in the final image.

This image is the result of bracketing then selecting the 1 minute exposure shot with a wide angle lens at f11 on TMAX 3200 film.

IMAGE TINT : GallerySilverLite

The silvery tint was used to highlight the look of the cold textures in the snow, and support the overall look of the cold winter night.

This B&W image was tinted in Adobe Photoshop with an ICC Profile I generated from my Mac App SuiteProfiler. The Profile was derived from the “GallerySilverLite” Color Map created in SuiteProfiler.

Click these buttons to download the ICC Profile and SuiteProfiler Color Map:

EXERCISE : Light Intensity

Find a night scene lit by a single light source. Compose a shot to catch as many variations in light intensity as possible.

Bracket for a variety of exposures to make sure you capture the full range of light cast by the light source.

Be sure to review the Safety & Precautions page.

FEEDBACK : Light Intensity

Leave comments on this post to share your ideas and experience, or ask questions.

NEXT TIME : “Why Bracketing?”


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7 thoughts on “Light Intensity

  1. this picture makes me want to peek around the tree and see what is making the light. is there a dance going on behind that tree?

    • I like how you zeroed in on the impact the image had on you. In future posts I will address the subject of impact, and how it applies to nighttime image making.

  2. Excellent post. They all are, but this was particularly helpful to me. I’m not quite there in my understanding, as I’m math impaired, but I get the concept. Thanks!

    • The essence of this post was to draw attention to the dramatic effects of lighting we see at night. Light gradients like the one in my image is something we can hunt down and capture in night photography. The exercise is meant to help you bridge the gap between mental understanding and your own experience.

  3. I hate to make things more difficult for Rhonda. However, I want to comment that the image in this entry on light intensity is really great. Since essentially all of the light that makes this image so great is reflected light and not light coming from a source emitting light, the inverse square law just is not complete as an explanation of the intensity of light reaching the camera lens. Reflections really complicate things. Sometimes we are dealing with light that is reflected more than once. We may not think of indoor photography at night using available light as night photography, but we are still dealing with artificial light sources that are a lot closer than the sun. Indoors light is reflected off of walls, ceilings, floors, and objects in the room onto objects in the room, walls, ceilings, and floors. The brightest part of this image is the ground (covered with snow it seems). Light reflected from the snow onto the trees is illuminating the trees to some extent. However, the light reflected from the snow is rather diffuse meaning the mathematics of how much light reaches the lens that is reflected from trees and other objects that was first reflected from the snow is really complicated integral calculus. I am certainly not denying the validity of the inverse square law. However, because of the complications of secondary reflections I doubt that if the camera had been moved closer to or further from the primary light source obscured by the closest tree that the inverse square law would have allowed us to calculate precisely how much light would reach the lens. We would still need to bracket or depend on the kind of intuition that Dana has in doing night photography. I would like to add a disclaimer. I am sure that I did not put as much thought and time into this comment as Dana did into this blog entry.

    • In simple terms, there are light sources and reflected light. This is only complicated by the fact that light bounces around. So the Inverse Square Law is not quite as pure as we might like.

      However, we can treat it as a practical rule of thumb (you know the 80-20 rule) with great success for both visualizing light and adding supplemental lighting to a scene. We really do not need to calculate all of the subtle nuances of secondary light reflections.

      There is also an intermittency effect that can be applied as well, but I’ve found that in practice it is not worth worrying about.

      In general, we are only concerned with what light reaches our camera, not so much how it got there, or why.

      When we explore solutions for alternative lighting in future posts, we will refer to the Inverse Square Law, and obviously concern ourselves with the “how” and “why”, but only in practical and productive terms.

  4. Pingback: Fire | The Art of Night Photography

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