Controlling Light: How Many and How Much
So far we have centered on understanding and working with a single light sourcethe main lightand we have mostly considered available light. We’ll concentrate the remainder of this article on manipulating light by adding light to the scene rather than just working with what is already there. This dramatically improves our ability to control contrast, adjust shadows, and manipulate texture. It gives more options in setting exposure.
Multiple lighting can be as simple as the sun and a sandy beach, or a studio with banks of lighting equipment. Most multiple lighting applications only use two or three sources. Complex studio lighting is beyond the scope of this article, and priced beyond the budget of the average amateur. We can borrow concepts and methods that the pros use without high-powered strobesif we know how to use multiple sources. The good news is that simple reflectors and advances in small flash units can produce similar results, with far less cost and bother. The Naming of Names There are some terms to define. When there was only one light in the picture we could just say “the light,” or “the source.”
Photographers identify multiple light sources by what they do, as well as how bright they are and the source. That makes it easier to manage what effect the light is supposed to have, and how to set its intensity. One common way to name lights by function is borrowed from motion pictures. In this system, the primary light source is usually called the main light or the key light. This is the light that is used to keyor indexthe exposure setting and provide the majority of the illumination on the subject. That does not mean it is the brightest light source in the picture, or that it has to light up the entire scene. (Of course it will be the brightest light if there is only one light. Even then, the main light may not light the entire image.) The picture was made using what filmmakers call a key spot. The main light is tightly centered on the subject and much of the scene is in shadow or even totally black.
A fill light does just that. It fills in or reduces the darkness of shadows. It is never as bright as the main light, and is not usually calculated into the exposure setting. The most basic multiplelighting setup consists of the key light and a fill light.
Background lights are placed behind the subject to cast light back onto the background. Novices think that the color of a background is determined by its color. That’s only true if light falling on it is close enough to the exposure when the picture is taken. If the light falling on the background light blasts enough light, the background is recorded as pure whitejust like a very bright light is recorded as white in a picture. If the background is in shadow, or too far away from the light source, it will be recorded as black. The image was taken with a carefullycontrolled exposure. The cake is lit from one side, and the background from a source hidden behind it. The levels were set to let the candles form a bright ring and just illuminate the tablecloth.
Accent lights are used to add highlights to provide more exposure to a specific area of the picture than the key light, or to illuminate part of the composition that is not fully lit by the main light. They are often named by what they light. For example, a hair light is usually placed above and behind a subject to add highlights and sheen to the hair, and possibility to add light to visually separate the subject’s head from the background. A rim light works the same way as a hair light, but isn’t focused on hair. Gary Todoroff’s picture of Capetown sheep uses the sun as a rim light to outline a mother and her kid.
He set his exposure so that the open sky served as the main light, and positioned the camera so that rim light, combined with the morning haze and a shallow depth of field, separated the animals from their background. Bounce light casts indirect lighting off a surface like a ceiling, a wall, or a reflector. It can be a natural occurrence, or placed deliberately by the photographer. Bounce lighting is usually diffuse, unless it is reflected off a very shiny surface held close to the subject. News photographers often raise a flash close to the ceiling, creating a bounce effect that combines with existing overhead lighting. This approach produces soft shadows and allows for exposures with better depth of field.
There are two more definitions needed before we start using multiple lights, continuous and transitory. A continuous light source is one that stays on and can be seen while determining the exposure, like the sun or a light bulb. With a continuous light source, the longer the shutter is open, the more it adds to the exposure.
A transitory source is only on during the exposure. Examples include flash units, lightning, and fireworks. With flash, the light duration is so short that f/stop is the only real control over exposure when flash units are the only light source. That’s because the light does all its work in less time than the shutter is open. Be sure to check your camera’s manual for information on its flash sync speed. The shutters in DSLR cameras are only fully open all at once, below a certain specified speed. Below that specified flash sync speed, which ranges from 1/60th to 1/250th of a second, part of the image area will be covered by the shutter and not be recorded. Some flash units have the ability to fire several rapid bursts to overcome this limitation.
It’s All Relative to Intensity
The key (pardon the pun) to mastering controlled lighting is in understanding the concept of lighting ratios. The main light source is always used to determine the exposure. Fill lights are placed and adjusted to reduce the shadows, and are always less bright than the main. The difference, expressed in exposure f/stops, is the lighting ratio. For example, a 1:2 ratio means that there is a one-stop difference, and the light falling on the darker side is one stop less than the main light.
Here’s a simple example. You are in open sunlight and want to reduce the harsh shadows. If you use your on-camera flash as a fill, step the flash’s intensity to one stop less than the daylight exposure used for the picture. Your camera or flash probably has a feature that can be set in f/stopsthat’s easy. If the sunlight exposure is 1/200th of a second at f/11, then the fill needs to generate the amount of light needed for an exposure of 1/200th of a second at f/8. That’s a lighting ratio of 1:2. The first number is the main light, and the second is half of that.
A quick recap before we cover some lighting setups in detail. The main light is whichever light source is used to determine the exposure. That’s why it is called the key light. A fill light is used to make the shadows light and reveal more detail. Accent lights put more exposure on a specific portion of the image than the main light to create a highlight. The relative intensity of one light to another determines how they interact in the composition.