Working with Fall-Off
By Christopher Grey
Published by SekonicChristopher Grey explains how to create even illumination and how to control fall-off for creative purposes.
Photographer Chris Grey is an expert on lighting. He's written seven books on the subject, teaches classes and workshops, and is a strong advocate for precise control in the studio. To see more of his work, or to find his books and DVDs, visit his Web site.
One of the important factors in lighting is the key light’s distance from the subject. Light weakens as it travels away from its source. This is as true of the sun as it is of any other light. One way to express this is mathematically, through the inverse-square law. It says that the power of light is inversely proportional to the square of the distance from the source to the subject. In more practical terms, it means that if you double the distance between the source and the subject the light now reaching the subject will be only one quarter as strong, because the circle of its influence is now twice as large (and it has twice as far to travel).
Be aware that while this is a law of physics and therefore absolute, it is visually absolute only when dealing with one light source in a darkened room. Many other factors, such as ambient light in the room and spill from other light sources, may affect how much the key source is diminished.
Depth of Light
Tied directly to the inverse-square law is a little sub-truth we call "depth of light." In simple terms, it means that for every position of the source, whether it’s close to or far from the subject, exposure is constant for a certain portion of the distance across the subject. In really simple terms, the further the source from the subject, the more even the exposure, front to back, across the subject. Conversely, the closer you move the source to the subject, the faster the light will fall off. Your "depth of light" will become shallower the closer you get.
If you are lighting a group of people many rows deep, for instance, you will want a powerful light source set as far as possible from that group (it may be many feet behind you). You will need power, of course, so you can use a small aperture and get enough depth of field to keep everyone in focus, but also to ensure that the depth of light will expose everyone evenly from front to back.
Because strobes are quite cool in temperature, especially if you're using a modifier such as a softbox, you can place them close to your subject without worrying about igniting hair or melting polyester. The depth of light will be minimal, and the effect can be very interesting.
The key for the simple one-light scenario I'll show you below is a medium softbox. The subject is placed about 6 feet from the background, to pick up some spill and keep him from dropping off into shadow. I’ve placed the key about 6 feet from him and metered off his camera-left cheek, letting the shadows come. Whatever shadow detail there is is a result of light bouncing around the studio, as there are no fill cards in use.
In the image below, the key has been moved to about 1 foot from his cheek. Notice how the depth of light has decreased, but the shadows remain about the same.
The shot below was taken with the light about as close as I could put it—about 4 feet away—and still keep enough background for the composition. The depth of light is extremely shallow, but the highlights are bright and clean and the shadows are deep but full of detail.
I wasn’t kidding when I said the light was close.
Building a Fall-Off Portrait
To shape a portrait using fall-off, I set my background light first so that I could move my model into position when I saw how the light formed. There was nothing more than a 6-inch dish on the strobe, and it was placed about 6 feet from the paper. Inside the specular circle the exposure was too hot to read, but it read f/22 at the edge of the diffused highlight, so that is the power output I needed from the key.Category: How-ToClick for more articles of: Christopher Grey
With my model in position, I moved in the 18-inch dish and 40-degree gridspot to light his face, setting the key about 2 feet from him. The resulting show, below, is actually quite nice, but I wanted to boost the contrast even more and play light off the planes of his face.
The 18-inch dish dish was changed to a 6-inch dish with barndoors, which I closed until they were only about 1 inch apart. The background light was also lowered, and angled up to be dominant on the camera-left side. This placement defies conventional practice, which says that background light should always fall on the shadow side, but it added something unusual to of the image.
Something interesting happened when I closed the barndoors. The inside of the dish is faceted to distribute light evenly, and because the opening was so narrow, the barndoors acted like a small aperture (lots of depth of field), which projected its own shape in several places behind the subject. This was unexpected, but it added something extra to the image.