What Backlight Can Do for Your Photos
Backlight is probably the least understood component of lighting. When people start learning the craft, they easily understand key and fill, but often neglect the backlight. While it might not be as noticeable, it still has the power to influence the quality of an image.
A photograph is a two dimensional image. To create depth, you need to take advantage of depth of field. And to take your depth of field up another notch, you need backlight. Backlight creates a halo around a person or object which helps pop them out of the image. It adds value to your shots.
How to Create Backlight
Because backlight complements the lighting, you don't need a big gun. In fact, you want a small light instead of a large one because you want the beam to be narrow. You want to set the backlight just out of your frame, behind the subject. You want the light to be set at full spot so that beam is hitting the back of your subject.
You don't want to use a large light because that will create spill on the sides of your subject and instead of backlight, you'll have a big light source coming from the back. If you do need to use a large light, then make sure you have flags on hand to cut the spill so that the only light that makes it out of the lamp hits the subject's back only.
You could also use a snoot to make sure the path of the beam stays narrow. If you don't have one, you can quickly make a makeshift snoot out of black wrap.
Because backlight faces the camera, there is a high risk of lens flare. Lens flare happens when a light source directly shines into the lens. If it does occur, then you need to set a flag out of frame, half way between the lamp and lens, to block the light from hitting the camera. These are called lensers.
The purpose of backlight is to highlight the subjects and have them pop out of the image. Backlight helps make an image seem more three dimensional and it also directs the eye's attention to the subject. You want backlight to be noticeable, yet subtle. It should only be identified by a viewer when they're consciously looking for it. Too much backlight runs the risk of looking cheesy and fake if it has no purpose. But, if you're using a lot of backlight for creative purposes, like emphasizing someone is divine, then you can use backlight to suit your story.