Lighting

With direct sunlight on the subject the background can be seen as normally exposed. In this case the exposure was 1/250 sec at f/8.5 at ISO 100. Photos © 2001, Tony L. Corbell, All Rights Reserved

People who have attended art classes have been taught that in order to produce depth in a painting there must be a foreground, middle ground, and background. Control over this dimension and depth usually is the result of planning and foresight. As photographers we tend to often overlook the foreground and generally allow the background to fall wherever it wants. Controlling the background is an area of photography that can be mastered as long as the photographer learns to think about what the scene will look like after appropriately using a light meter to establish the true exposure of the subject.

Shifting The Key Of A Photograph
Changing the exposure or brightness of a primary subject area in order to effectively raise or lower the exposure or brightness of the background is a technique known as keyshifting, literally shifting the key of a photograph higher or lower based on what happens to the primary subject.

The background exposure remains the same when we place a diffused translucent panel between the sun and the subject, decreasing the exposure on the subject without compensating for the loss of light on the subject. Exposure remains at 1/250 sec at f/8.5.

If you've ever seen a movie or video shoot, you've noticed the crew raising very large scrims of a translucent material. The purpose is to allow light to travel through the fabric in order to control the quality of light on the subject and at the same time elevate the brightness of the background. The loss of light caused by the fabric requires an adjustment of the exposure. Video crews also use large lights bouncing into silver panels designed to brighten the light on the subject in order to darken the brightness in the background. By lighting the primary subject, background control is easy to achieve.

One of the easiest ways to understand the effectiveness of keyshifting is to place a subject in bright sunlight and make an exposure. The subject and the background will have an equal brightness level. If you place a translucent material between the subject and the sun, the subject will receive less light. By adjusting the exposure for the loss of light on the subject, the background will naturally get brighter.

As we open the aperture to correctly expose for the new exposure on the subject, the background shifts brighter by the same amount of light that the fabric decreased on the subject. In this case, the exposure difference was 1 1/2 stops or f/5.6.

In terms of what the film sees, taking light away from the subject effectively adds light to the background. The more light you take away, the brighter the background will appear. In this case, in addition to changing the background you are also changing the quality of light on the subject. Diffusing the direct sunlight softens the shadows and lowers the brightness of the highlights.

Understanding how to best use this simple technique causes a photographer to look at backgrounds differently. I will often look at a particular area that will be my background and try to visualize what it will look like if it is overexposed by one or sometimes even two stops. This will help me if I know I am going to place a translucent disk or panel between my subject and the sun to change or possibly improve the light quality on a face.

This photo was taken with an exposure on the subject of f/11. The exposure of the background was also f/11.

Thinking Like Film
One of the first things photographers should try to understand is that while metering for the brightness of any background in a studio or outdoors in the park, the final exposure of a photograph will almost always be based on the amount of brightness on the primary subject. This may or may not be in the same light as the background. In many cases, they are not in the same light. This means there are two options--to brighten or darken the background. Either add light to the subject or take light away from the subject.

A photographer who has full control over his or her environment will take advantage of this technique by previsualizing the scene and thinking about how it will appear on film, not simply to the naked eye. Always keep in mind that our eyes will fool us into seeing one thing while the film sees another.

In this image, I reduced the output of the main light on the subject by one stop (f/8) and made the adjustment on my lens aperture. This exposed my subject properly, yet the background appears brighter.

Background Control In The Studio
I learned how truly effective and important this lesson of control is when working in the studio. When I wanted my background to be brighter, I discovered that the background light was on its brightest or highest power setting. Since I could not add light to my background to make it brighter, I finally figured out that I could simply subtract light from my subject. By then adjusting the exposure--opening up my lens aperture to adjust for the loss on the subject--the background appeared to get brighter. Again, the more light I take away, the brighter my background appears.

The converse is also true. If you need the background to be darker it's a problem if your background light is on its lowest power setting. By thinking about shifting the key, you can easily increase the brightness of the lights on your subject. This will increase the subject brightness. When you close down your aperture for the increase of light, your background gets darker.

By reducing the main light output by another stop (f/5.6) and opening the aperture one stop to again keep the subject properly exposed, the background shifts brighter, by another full f/stop. The background was still receiving the same amount of light as shown in the other two frames.

In applying this technique to your own photography, try to remember it is as an exercise in thinking more than anything else. It is all about thinking forward to the final result and taking charge of your picture.

Tony L. Corbell is a long-time professional photographer, educator, consultant, and author in the photographic community. For more information, contact: www.corbellproductions.com.

Technical Equipment
Camera: Hasselblad 555ELD
Lens: 250mm CFi Sonnar
Film: Fuji Provia 100F (RDPIII) at EI 100
Lights: Three Visatec 1600B Monolight units