Control Over Your Lights With Distance
This is part 4 of the learning to light series with Josh Larkin.
In my article on Apparent Light Size, we saw the differences in light produced by large and small light sources. I demonstrated this by starting with a flash in a small softbox set up very close to a small object, therefore making a large light source. I then moved the light farther and farther away, thereby simulating a smaller light source. The actual light size didn't change, but its apparent size did in relation to the subject. I noted at the end of the post that as the light source got farther away, the background began to get brighter. The reason for that is something that I've struggled with in learning to light, and it's called the inverse square law.
Now before you go surfing away I'm going to put out there that math and me are not friendly. Seriously, I jumped the shark in tenth grade algebra, so the minute this little aspect of lighting came up it was a challenge for me to grasp. Fractions. Equations. Physics...I just wanted to make photographs.
But here's what I'll tell you. Read about the inverse square law here and on the web and don't let it confuse you. Just get familiar with it. Then bust out your camera and flash, set up somewhere where you have a good working distance, and start shooting. You'll get it. I promise. Why? Because if you leave the fractions for the academics to ruminate over and just pay attention to what you're seeing in your photographs, it will make sense.
The gist of it
By definition, "an inverse-square law is any physical law stating that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity." Huh?
Well, for you math-minded folks, that probably makes sense. For the rest of us, what that means is that there is a way to calculate how much light fall off occurs as a subject moves farther away from a light source.
Now I'm not going to try and explain the math behind this, but the basic idea is that if a subject is one foot from the light source, the subject will receive 100 percent of the light's intensity - the square of one is one, and the inverse is 1/1, or, 100 percent. Move the subject two feet away, doubling the distance, and now they receive 50 percent of the light's intensity. Ha, fooled you. This is where the inverse square law shows up. The square of two is four, and the inverse of four is 1/4, or, 25 percent. So that one-foot difference means you lose 75 percent of the light's intensity at your subject's position.
But of course it's math, so it gets a little bit more complex than this. That first one-foot change in distance equaled a huge loss of light. But that rate of loss isn't constant. In fact, the farther you subject gets away from the light source, the less fall off you have between two distances. For example, at 9 feet from the light, the square is 81, the inverse is 1/81, and rounded to the nearest whole number you have 1 percent of the light. Move the subject to 10 feet - square is 100, inverse is 1/100, rounded to the whole number is....1 percent. The diagram below shows the rounded percentages of fall off for subjects positioned between one foot and ten feet from a light source (disregard the actual light color, I didn't calculate the proper gradient steps).
From a practical point of view, if you're photographing a subject at one foot from the light source at f/16 and he or she moves a foot back, your subject will be drastically underexposed, and to correct that, you'd use a larger aperture, say f/11. That will get you the additional light you need. Now, if that subject is at nine feet and you're at say f/4, if he or she steps back to ten feet, there's very little difference in light intensity, and your f/4 aperture can stay the same to get a correct exposure.
Imagine now photographing a group of people in two rows. If row one is positioned one foot from the light, and row two is somewhere between two and three feet from the light, there's a one to two stop difference between correct exposure for the front and back rows. The solution here would be to move the entire group back to the eight to ten foot from light source area, thereby equalizing your exposure between the rows.
Below is an example of five photographs of the same subject lit with an SB600 fired through an umbrella at 1/4 power. Shutter speed was 1/250, ISO 100, aperture at f/4.5.
So, from one foot away the side of the subject closest to the light is properly exposed. At two feet, there's a quick drop off to underexposure. At four feet, it's very underexposed. At 8 feet, you can barely make out the subject, and the same applies at 10 feet. And that's the crux of it. The difference in exposure between eight and ten feet is barely discernible, as opposed to the difference in exposure between one and two feet, which is very noticeable.
Getting control of the situation
So an understanding of the inverse square law can help with photographing groups. Got it. Well, if you've been following along and thinking through this idea that exposure close to the light source is vastly different from that farther away from the light source, maybe a little light went off in your head as to how you can use this to your advantage in different situations as well. Essentially, you can think of the distances between your light source, your subject and your background as a set of variables to tinker with to change different aspects of your photograph.
For example, the photo of our winged goddess to the right; was shot at 1/250th, f/9, ISO 100. I positioned the subject two and a half feet from a very light, almost white background and lit the scene with an SB600 shot through an umbrella at 1/2 power about one foot from the subject at camera left. At camera right about a foot from the subject is a white poster board-providing fill.
In this set up, our subject is getting approximately 100 percent of the light's intensity. The background, which is more than twice the distance from the light source as the subject is, receives only about eighteen percent of the light. As a result, the background is underexposed by a stop and has shifted from white to grey. Easy-peezy, we just turned a white background to grey by moving it slightly farther away from the light source.
So, white to light grey in just a couple of feet. The next shot was taken with the same camera and flash settings, same light and fill card position, but the background was moved back ten feet from the light source. The subject is still exposed correctly, since nothing has changed there, but the distance to the background is now so great that only one percent of the light intensity is reaching it. In fact, that's now a dark grey, so dark that I could make it go all the way to black with a little jiggering of the flash power and aperture.
Now here's the kicker. I started this post out with the lead image of the winged goddess in front of a bright green background. I did this to demonstrate what an understanding of the inverse square law gives us as photographers, even if we don't fully grasp the physics part of it. By learning to light the subject plane using distance and light position, we can then handle the background however we choose. For this image, I didn't change anything from the previous image in which the background is dark grey. All I did was add a second flash positioned at camera left about three feet from the background and fired it through a dark green gel. And just like that I've added a much higher level of control over the lighting in the scene, and it all started with a basic understanding of a very complex law of physics.
Read all articles in this series:
- Learning to Light
- Get Your Flash off Camera for Better Photos
- Apparently, size does matter...
- Control Over Your Lights With Distance
- Reflections
- Specular Highlight Control
All of our Speedlight and Lighting articles
We have many resources specific to Nikon flashes and suggest you have a look at the Speedlight index and the Lighting articles index.
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Originally written on November 2, 2012
Last updated on January 24, 2021
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5 comments
stephen sheoskie (sheo2859) on August 29, 2015
The visual diagram really opened my eyes to the fall off,great job.
Richard Luse (DaddySS) on November 10, 2012
Thanks Josh. Very helpful.
User on November 6, 2012
These are very helpful postings Josh. Having just attended a one day Kelby course called One Light Two Light by Joe McNally, your additional detail really puts the icing on the cake so to speak. Looking forward to the next installation Doug
User on November 6, 2012
Good to hear, Dale. I was wondering how the workshop went for you. ~Josh
Dale Maas (marnigirl) on November 6, 2012
Thanks Josh, Things are beginning to sink in. Your tutorial dovetails with a class I took over the weekend at Arizona Highways Magazine. Between the two, I am actually beginning to get a 'flash' of understanding!! Dale