Light Depth

Inverse what?

In our last exciting episode, I wrote about adding diffusion to your lights. In that article I made a few references to the depth of your light and why you want to have various size light modifiers instead of moving your lights closer or farther from your subject. I promised that I would write more about this in a follow-up article, so here we are.

As noted previously, the quality of light (hard or soft), is determined by the size of the light as seen by the subject. When first starting out, you might only have one light modifier, such as a 24x30 softbox. So the temptation there is to move the light in closer or move it back farther from your subject to control the size of the light, and thereby the quality of the light. This does work. However, it has a secondary effect on your image. Moving the light changes the depth of the light.

Depth is one of the four main things I think about when designing my lighting for a portrait. The others are the depth of field that I want (how much is in focus), the size of the light (the shadow edge quality), and the amount of power I need (to get the aperture I want for the depth of field I want).

The depth of the light is based on a law of physics called the Inverse Square Law. This law states that a specified intensity (in our case, light) is inversely proportional to the square of the distance from the source of that light. As an example, if you double the distance between the light and your subject the amount of light falling on the subject will be 1/4 (at twice the distance the light has to spread out to cover 4x the area, see the illustration below). If you move the light back to 3 times the distance, the amount of light will be 1/9th the power, and so on.  

inverse-square-1
inverse-square-simplified

Another part of this law, that seems to be often forgotten or ignored, is that it applies to a point source of light, something we rarely, if ever, actually use. As you put modifiers on your lights, such as softboxes, umbrellas, or add a lens, such as in a spot light, we start drifting away from the law. So don’t get hung up on the inverse square law or on measuring the distance between your light and subject. You only need to remember one simple rule: The closer your lights are to your subject, the darker the background will be.  

OK, sounds simple enough. But it is also very powerful knowledge. With this knowledge you can take a simple white seamless paper background and make it appear white, black, or any shade of gray between them. It also allows you to light your background separately from the subject to control the apparent depth of the scene. More on that later.

So, let’s look at some examples. In this series of images the subject is 65” in front of a white seamless paper background. The single light is a 24x30 softbox. The only changes between the images is the distance from the softbox to the subject and the power setting on the light to maintain the same f/4.0 exposure. As indicated in the images, the distances are 12”, 24”, 48”, and 96”. The first thing I think you will notice is how the tone of the backdrop changes. 

light-depth

You will also want to take a look at highlights on the subject. Look at the cheek. Specular highlights are mirror-like and at first seem to not follow the inverse square law. In actuality they do follow the law, but in a reverse way. Specular highlights maintain the same brightness, no matter what the light to subject distance is, but they change in size inversely proportional to the distance. As your light is moved farther from your subject that specular highlights get smaller and appear brighter because the rest of the image is getting darker while the highlight stays the same brightness.

specular-highlights

Now, about lighting your background separately from the subject. This involves adding more lights. In this case I added one light on my white background, a flash with a 7” silver dish reflector with a red gel. With the main light (the light on the subject) 96” away from the subject you can make out a pinkish glow on the background. But when I move the light in to 12” away from the subject the white background goes to a dark gray (as shown above) and the red gel on the background light can now be seen. If I had used a dark gray or a black background paper the red would be a richer tone than the pink that you see here. The background light was on its lowest setting. If I could have lowered the power more, it would have been more red, too.

background color

A practical application: Group Photos

So far, I’ve been talking about light falloff from subject to background. We can also think about group photos and fall off from subjects different distances from the light. By combining our knowledge of light falloff with a lighting technique called feathering we can learn to light a group of people. Feathering is when instead of pointing the light directly at our subject(s) we point it a bit off so that the light skims across our subjects. A fill card (white foamcore or a pop-up white disc) helps fill in the shadows from the right side.

group-lighting-1

As an example, if our light is on the left side of our camera we would point the softbox or umbrella at the person on the right side of the group. The main intensity of the light will now be aimed at the person farthest from the light and the lower intensities from the edges of the light will be lighting the person closer to the light, helping to even out the light across the group. If the light is still too uneven, we use our knowledge of inverse square to tell us to move the light back, farther away from the group to even out the illumination across the group.

group-lighting-2

Here is a group photo of my friends at CreativeLive posing for me. This image was lit with one studio flash in a 60" Photek Softlighter on camera left and a fill card on the right. The Softlighter was backed up a bit to help even out the exposure across the four individuals.

softlighter-group-photo

That's all for today. Thank you for following along.

JohnC