The Inverse Square Law. Sounds intimidating and if you read the actual definition or description of it, it certainly can be! However, simply expressed, it is nothing to be afraid of and, in reality, something to embrace if you want to take magical pictures using just one flash. So what is that simple expression of the Inverse Square Law? Just this: the closer a light source is to your subject, the faster the power or intensity of that light falls off to practically nothing. That is by no means an “official” definition, but merely my version of a simple one. What does it mean to you as a photographer shooting a subject with just one speed light? It means that you can turn the background behind your subject totally black even in a well lit room. No black background material needed. Magic!
I know how this works and yet, every time I do it, I am amazed. And it is so much fun to show it to others, because they can’t believe it. I hope you will give it a try. What do you need? You will need, of course, a speed light that you can place off-camera. You will need to be able to control the firing of the speed light from you camera either using your camera’s built-in, pop-up flash or a radio trigger. Naturally, you also need something to hold the speed light, but that can be the little plastic stand that comes with most speed lights or, if you have one, a regular light stand.
Once you have your light working off-camera, then it is all about placement of the light relative to your subject. For my example I used an arrangement of fall flowers. I placed the light, which I do have on a light stand, at about 80 degrees off-axis. On-axis would be where the camera is, i.e., right in front of the subject. So, directly to the left of the subject would be 90 degrees. Since I want light to hit the front of my subject, I moved it from 90 degrees to 80 degrees to make that happen. The classical starting position for one light is 45 degrees, but that would throw more light into the background and that is something I am trying to avoid. As you can see in the accompanying behind the scenes shot, I also have it raised and pointing down at the flowers at about a 45 degree angle.
Next, I make sure that the light is very close to my subject. In my example, I have it approximately two feet (61 cm) away from the flowers. This is where the Inverse Square Law comes into play. Since the light is only two feet away from the subject and pointing mostly off to the camera-right of the subject, very little light from the flash is making it to the background. In this case, that is a brightly lit living room in the middle of the day.
The final factor that helps make this magic happen is the shutter speed I have chosen on my camera. I have my Olympus OM-D E-M1 shutter speed set to its sync speed, 1/250. When shooting with a flash, the camera’s shutter speed controls how much of the ambient light (i.e., the light in the bright living room) is registered on the camera’s sensor. At 1/250, no ambient light is registered at all! I can prove that to myself by taking a shot with no flash but with my camera at the settings I will use when i do turn the flash on. Those settings are 1/250, aperture of f/11, and an ISO of 200. I turn off the flash and take a shot. When I review that shot, the frame is totally black. When I do take my shot with the flash turned on, then, I know that only the light from the flash is what is illuminating the scene. I generally shoot in Manual Shooting mode (on camera) when using a speed light.
Pretty amazing, don’t you think? I do. And if you would like to learn more about how this works and practice doing it yourself, get in touch and let’s talk about doing some tutoring sessions using your camera and speed lights. Or, if you don’t have any speed lights, you can use mine to see if this is something you want to explore further before spending the money on speed lights. This can be done, by the way, with inexpensive equipment that includes a $38 speed light and $25 radio triggers.
NOTE: Equipment I used to create these images - Olympus OM-D E-M1, Cactus V6 transceivers (radio triggers), a Canon 580EX II flash, and a Cheetah light stand. I used my iPhone 6 for the camera settings and BTS shots.
If you would be interested in virtual tutoring sessions using either Skype or Google Hangouts, please drop me and e-mail or leave a private message to me on Facebook or Google+.
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