The Inverse Square Law can help us to understand how moving the light closer or farther away can consistently affect the brightness and depth of light on the subject. The position of the light unit, however, does not only affect the subject, but also everything past it, which can be either lit or in shadow depending on the amount of fall-off affecting the light.
By simply changing the distance between light source and subject, then, we can alter the light fall-off to our advantage, enhancing it or minimising it to control the amount of light reaching the background.
This simple technique is a fast and efficient way to consciously choose brightness and the shade of colour of our backdrops to make them as suitable as possible to every different project and image, reinforcing the image concept and visual impact.
Remember that the overall brightness of your subject and background is also affected by any ambient light in the area where you are shooting. Use your shutter speed to control how much of this light reaches your sensor and affects your images (fast shutter speed to minimise ambient light, slow shutter speed to enhance it).
To lighten a background
The main light is equipped with a beauty dish and positioned about two metres away from the subject. With the light unit far away from both model and background, the fall-off effect of the light is highly minimised, allowing it to reach the subject evenly and brighten up the background behind them, making it appear in its original shade.
- Use large light sources
- Move the subject close to the background and the light far away from both.
To darken a background
In this image the same light unit has been moved much closer to the subject, now about 30–40 cm away. (The intensity of the light has been slightly reduced to be able to produce the same exposure as before, so that the two images could be correctly compared.) Since the proximity between light and subject has now considerably increased, the fall off of the light is greatly enhanced. The light on the subject is much less even than before, already starting to fall-off past the model’s left cheek and shoulder. By the time it reaches the background, the light has already lost most of its power, to the point that the backdrop now appears to be a much darker shade, making the subject stand out.
- Use small light sources
- Move the subject far from the background and the light close to the subject
- The darker you want the background to appear, the further the subject should be standing from it and the closer the light should be to the subject. These can make a white background look grey or even black, or hide it in shadow.
- Model: Anna Tatton
- Makeup and Hair: Virginia Bertolani
- Shutter Speed: 1/160
- Aperture: ƒ/5
- ISO: 100
- Lights: Studio head 1 with beauty dish (front, camera left)
Lighting People: A Photographer’s Reference is Rosella Vanon’s complete reference and guide to lighting techniques and posing models. Lighting People is an art every photographer—and every photography student—must master, and this single volume is both a complete course and the most useful reference book you can find.