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.
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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